I'm blessed (cursed?) with a very long DIY conversion of a duplex to a single house... the "blessed" part is the 2nd kitchen has been stripped of appliances but has a large 1930's white enamel single sink (will easily take a 16x20), and working plumbing.
But the way I handled the door may work for you if you have a decent sized closet... I bought a end-of-season close out blackout curtain from eBay (dark blue but 100% opaque, fifteen bucks) and installed a thin curtain rod. Then I made sort of a "valance" of black felt which is stapled around the top of the doorway and hangs down about 8" over the curtain & rod at the top. I fastened the curtain to the doorway at the hinge side, and it's long enough to go past the door and curve into the room at the corner. When you close it up, you have to sort of "tuck in" the corners, but it's completely light tight for almost free.
In the past, I've glued black felt into door jambs to seal any secondary light leaks, and added a "door sweep" (rubber sorta squeegie looking weatherstrip) to the bottom.
You might want to put in a waist-high shelf or small table in a closet situation. I find it a lot easier to load film on a table than in a bag.
I did my windows from my standard commercial studio gear... black seamless paper and black gaffer tape.
The idea mentioned a few posts above - staggered darkroom doors - is effective if everything is flat black, but you need several feet for that to work.