I have not done it myself, but one tip is to understand that a retroactive forbearance is strictly up to your servicer. Specifically,it's usually whoever you talk to that can put in the change. You're basically asking, nicely, and hoping that you will eventually get a sympathetic ear (or eye) that will grant it to you. I would recommend against spamming them. I've read quite a few stories where people just called until they finally got someone who was feeling generous.
There's a few recent threads and I think they explained how they did them. Hope this points you in the right direction.
