I used to be a regular practitioner of Brazilian Jiu Jitsu. When I started out it took me a very long time to be able to sit on my heels for more than a few seconds. It was uncomfortable on my ankles and also led to a lot of foot cramps. Historically, it has been an issue in Yoga classes too (usually Vajrasana/Thunderbolt Pose), but it's not as central to a typical class as it is for the martial art class.
After years off, I'm thinking about making a return to the sport in the coming months. I did a few experiments and I feel like I'm back at square one for ankle mobility after trying to sit on my heels a bit.
Aside from just sitting on my heels for increasing lengths of time, are there any exercises I can do to prepare my ankles? It took months of class before I was comfortable when I first started.