I find a very useful thing to have is a small stainless steel square - this sort of thing
https://www.newmodellersshop.co.uk/e...eel_square.htm
You will of course have local sources.Its an enormous help finding out which bit isn't quite square, and its also a handy small straight edge.
Rather than sanding, for this sort of thing I favour scraping with a Stanley knife blade held at 90degrees to the surface. My sanded surfaces have a distressing tendency to acquire a dome shape (more skill would presumably prevent this) and I find it much easier to keep the surface truly flat scraping - and also easier to square up the not quite square.