After being home for almost 9 months on medical release...
My process of returning included:
-Getting written permission from my doctors saying my body was healthy enough for missionary service
-Meeting with my Stake President
-Writing a letter stating my desire to return sent with a letter from my Stake President and the note from my doctor to the missionary department.
Then a couple weeks later I got a call from my Stake President saying I was approved. They extended me a day to return, then a week later I had a mission call type letter (but much shorter) saying I would be returning on March 24th.