You can refer to this blogpost here which has pictures and is where I got all my knowledge. I'll simplify the steps here for those that are impatient and add my comments of things that I ran into later.
This was probably the hardest part. For those of you that don't like rants then you can skip down to the list of materials, but this really just infuriated me over the close shop mentality. I don't know of any "Cobbler License" out there, and I guess most of the "Cobbler Suppliers" don't either, so they came up with a different practice, only selling to licensed medical professionals. Nearly every store website I went to that carried Birkenstock soles fell into one of two categories. Either they were A) closed or B) only sold to podiatrists or diabetic specialists ( I assume due to the lack of circulation in their feet a lot of diabetic specialist also second as cobblers or have cobbler friends that need to make specially designed shoes for their patients).
There were a couple of other sites, but you had to open a credit account and have a business license in order to order from them, that made it a bit more difficult than I was willing to do. It pretty much seems like you have to already be a cobbler to purchase cobbler supplies.
Another option was purchasing from a cobbler. One in my area wanted about $35 just for a pair of soles, or $65 to put them on for me. I did find one website after about a week of searching, but he was hesitant to sell the remaining of his Birkenstock sheet (less than or equal to one sheet), but offered to sell his Vibrem sole sheet for $75.It was a stroke of luck that the next day I ran into a Canadian website and was able to purchase a single sheet (although the sales representative was a little surprised when I only ordered one). There was a second site in the UK that would have been about the same price.
Here's the list of material:
That should do it, if you bothered to read my rant you might find the soles are the hardest piece to come by, everything else was relatively easy to obtain.
You might want to take some considerations for your oven and the food that you'll cook in it later. I opted to place a sheet of aluminum foil under my sandals to catch any dirt. Also, after removing my soles I also used the self-clean feature to burn everything else that might have been left and turned on the stove fan and opened a couple of windows. There wasn't very much smell the next day.
While the oven is heating you can begin other preparations as time permits. You'll need a place to cut, either a utility bench or a cutting board will due.
First you'll want to place the sandals on the material, being the scrooge that I am I tested out various patterns that would yield the most soles and opted for the L and R facing opposite directions next to each other. I estimated I could make about 10 pairs, or 20 soles, this way. Try to outline a little larger than the actual sandal, it's easier to cut away extra material than it is to glue more on.
The material can be a little tough, but I found that with a utility knife making a shallow sketch and following it over and over works out nicely and is easier to control. There is a grain to the material, so I recommend making the first cut along the grain and then you can bend the loose material down while doing multiple cuts against the grain, this will make it easier to hand the second edge.
This step isn't absolutely necessary but I believe it is beneficial. One thing I tried to do was soak the rest of the sandal to help protect it from any heat damage done by the oven. It may have been a good idea to add lotion to the leather beforehand, but having not done so I can't say that the results would have been any better. I did notice that after baking my leather is darker next to my unbaked sandal.
You can go ahead and place your sandals in the oven and set a timer. I placed a sheet of aluminum foil directly underneath mine to prevent any dirt from falling down. It would also be a good idea to turn on the stove's overhead fan and to open a couple of windows nearby unless you love the smell of burnt rubber. This rubber is going to shrink, and that's how you know it's working.
During this time you might want to prepare your workstation for the next step. You'll want to be able to work quick once your sandals are out so you don't have to bake them twice. You'll need your butter knife or whatever you've decided to help with prying the sole off. It's going to come off easily, and the knife may not be necessary, but it's better to have something at hand than to run and get it while racing against the cooling clock. I'd also recommend having the utility knife at hand which you can use to "cut" the strands of adhesive while prying the sole off.
Now the time has come to see if we really are crazy like my wife kept asserting. Although we're used to running our ovens at 350F and higher 200F is nearly hot enough to boil water, and definitely hot enough to give your skin a burning sensation. Before you remove them from the oven you should have already prepared your workspace and have all tools that are needed nearby, namely the butter knife or other flat edge to help in prying the sole from the rest of the sandal.
Remember that your sandal is delicate and you don't want to ruin the base between the cork and the sole, you should have noticed that the sole has shrunk while in the oven, time is of the essence, you want to work fast, but don't be hasty. Slowly peel back the sole and you can use the butter knife to encourage it a little bit by placing it near where the sole is now separating from the sandal. If you're taking too long it may get harder towards the heel. If peeling from heel to toe it may get harder towards the toe.
This step may or may not be needed, if someone could leave a comment arguing either way I'll incorporate it into any edits I do to this page. For me, my experience was not really having a good thing to roughen either the sandal nor the sole, whatever I did try using didn't seem to do anything. You can probably just skip this step
You should have already prepped your soling material by cutting it into roughly the shape of your sandal or at least in a large enough rectangle that you can comfortably place the sandal on and not be lacking in material.
I applied the cement by first doing a zig-zag pattern from the toe to the heel and then using a plastic spoon to smooth out the cement. Make sure that you get every inch of your sandal, especially the outer edge.
Once the sandal has been done you can now apply the same to the soling material. Do a zig-zag pattern and spread with the spoon, make sure that it extends past where you think your sandal will be to ensure that your entire sandal will adhere together.
DO NOT put the sandal and soling material together right away!
Follow the directions on the box or bottle for how long you should wait before pressing the two pieces together. Mine was around 15-20 minutes. I set a timer and started reading a book.
After having waited the appropriate amount of time I then went about putting the two pieces together. This must be done with great care because once you have let the cement dry slightly it becomes extremely adhesive and once in touch with itself it will be nearly impossible to let go, this is the primary reason for using a piece of soling material that has not been precisely cut, therefore allowing you a little wiggle room if you happen to be slightly crooked.
For me I started with the corner, if you can call it that, where the straight outside edge curves to form the toe area by eyeing the straight edge and slowly put them together. I'd like to emphasize again that this is extremely adhesive and you don't really get a second try.
After this is all said and done you now have a successfully put a new sole on your old sandal. But don't start celebrating yet by wearing them around, you still need to allow the adhesive to cure for the next couple of hours, preferably all not. It is a good idea to use some sort of weight to keep the sole and sandal together from toe to heel. For me I used the leaves of my table although I feel that it wasn't quite adequate enough at distributing the weight.
Now that the adhesive has cured the sandal should be fit for wear outside the house, but wait! One final step, we have to cut off any extra material that is left. I used a utility knife for this. For the most part I just cut off in large chunks with straight lines. There was some fine work to be done with the heel and toe, also I could have gotten closer on the sides if I had some sort of sanding tool like a dremel. I haven't actually done this yet, I've been content enough with just having cut off any protruding areas with the utility knife.
After this I realized that the bit I cut off from the heel was actually about the same amount I needed if I were to attempt a partial sole replacement on my left sandal. This was a similar process but instead of baking in the oven which would ruin the entire sole, I used the stovetop to apply direct and more localized heat after having scored the sole first. A spoon or some other wedge is extremely handy here. Once the portion of the sole you do not want is removed follow steps 7 through 12 just like for a full sole replacement.
A single sheet seems to have enough material to do at least 18 full sole replacements with about 18 heel replacements. Seeing how this took me about 4 months to wear through one side while doing what I would consider a light amount of walking this should last me around 3-6 years. But since learning partial sole replacement techniques I may be able to extend that out much further.
It's up to each person to decide whether to enlist the service of a cobbler or not. For me I felt that the price wasn't worth it, seeing how a half sole replacements is $10, so for the one and a half soles I did it would have been $30 a probably a bit more professional looking than mine. The Birkenstock is probably one of the easiest to resole though, and if you have the extra time and can find some soling material I feel that it was well worth it.