An ankle impingement can be anterior or posterior. While these stretches & exercises are mainly to help with an anterior ankle impingement, they can also help with a posterior ankle impingement.
The first thing to do is get the ankle loosened up with some manual stretching. You can do this by crossing your leg over and stretch it into dorsiflexion, plantarflexion, and eversion.
Then you will go into some stretches using your body weight. This will put a little more pressure through the ankle into dorsiflexion and plantarflexion.
After you have stretched the area, you want to strengthen it. Using resistive bands are very effective for strengthening the ankle. You can strengthen in dorsiflexion, plantarflexion, eversion, and inversion.
Finally, you will do an ankle mobilization movement with a strap. This helps mobilize the actual joint to help decrease scarring or adhesions, and it helps improve motion.