hello friends! I resoled my own birkenstocks. here's how I did it.
first, I went to ebay and bought some dogbone EVA sole material. You can buy precut shoe-sized soles or get it in a sheet and cut it yourself. I found a sheet to resole my and my boyfriend's soles for (I think) $40.
next, use a heat gun to heat up the glue holding the old sole on. I used one at ~ 420°F. It's helpful if you have a big clamp or something to grip the shoe so you have 1 hand for the heat gun, 1 hand with pliers to peel the sole off as you heat it. I did burn myself so be careful. I've also heard you can put your shoes in the oven to do this but I haven't tried it, so I'm not sure how well that works. With the heat gun, I didn't need to sand anything down as all the sole material came off relatively easily with the heat.
Once the old soles are off, patch the cork. I went to goodwill and found several cork squares for a few dollars. we didn't even use half of 1 square. crumble up the cork into smaller pieces. I had one shoe where I'd worn the heel down into the cork, so I used contact cement to build it back up. slather on contact cement, sprinkle cork crumbles, poke it around with a toothpick to cover the crumbles in contact cement, let it sit for 5 min to get a little more tacky, then stab it into place with the toothpick. You can also use a hammer or something to pound the cork into place and make it a sturdier patch job.
Once your cork is resoled, time to glue your new sole material on. Cut yourself the new soles in the shape you need (doesn't need to be perfect at first) with a box cutter or long handled razorblade type thing. I'm blanking on the actual name. Spread contact cement on your new sole (flat side) and the cork of the shoe you just rebuilt. Let it sit for 5-10 min until it gets tacky, press them together and use as many clamps as possible to clamp them together for like, 20min. I've heard it suggested to hammer the sole and shoe together here as well, although I did not. When you press the shoe and sole together and the contact cement doesn't move, it's good.
Even with clamps I didn't get a perfect bond between the shoe and sole, so I used more cork and contact cement to stuff into the little holes remaining with a toothpick. Then, remove the clamps and use a blade of choice to cut the soles to the shape of the shoe. Usually this is kind of messy on the sides, so I sanded it down at the end with some sandpaper.
And that's it! I let them sit overnight before wearing them. They are comfortable and feel much more sturdy now. If anyone's interested, I can link some youtube videos that I've seen of people resoling their birks as well. Hope someone will find this useful :)