@Guilean that's also not what he's looking ffor. Ues looking for a guide on how to backcourt addons that aren't compatible with ascension or even 3.3.5 possibly and backcourt them, like the great work szyler and his buddies did for dbm and elvui and whatnot.
And to answer the OP it's hard to make a one size fits all guide for this but make sure first off the .toc file is set to 30300 ofcourse.
Then I would say to make sure any references to wow API aren't either too new or too old and if any are adjust them accordingly.
And also very important is to make sure any of the XML frame elements or UI elements (if it has any menus or UI along with it whatsoever) is using tags that aren't out of date in either direction either.
I know this is an old post but I might do some work on backporting some simple not super time consuming to do addons that I could use, like arena stuff like gladiatorlossa or gladly I think would be easy to do and if I do get around to it I'd be happy to screen record the process and post it