Hey
I don't think a tutorial like that exists but I made a few window and door addons for different packs so maybe I can help you.
I wrote a few things about creating windows/ doors
here. I suggest taking a look at that post. It's a really rough explanation but it's more like a general guide what you need to do. If you want more detailed infos I'm happy to help.
When I edited the two tile door from Seasons and turned into a single door one I exported all meshes from the original door and deleted the parts I didn't need and then adjusted the frame to the size of a single tile door.
It's always helpful to look at EA meshes for a reference. Otherwise it's like adjusting any kind of object. The only difference is that you have a lot of mesh groups and you also need to adjust the wall cut out info and mask.
You can find a good guide for baking your own cut outs
here or try my method I described
hereIf you have any more questions feel free to ask!
