Some parts will need painting before assembly, like inside undercarriage bays, cockpit, Bomb-bay, etc, if they’re open for display. The crew need painting before assembly, usually best if they’re left on the sprue then the little bit that’s left unpainted after they’re removed just touched up to finish.
The main airframe should be glued, and any dried glue that stands proud carefully trimmed off with a modelling knife like an X-Acto, and sanded smooth with something like a nail-file or a needle file, then painted. Cockpit canopies and gun turrets need to be handled carefully ‘cos glue will fog them, especially if you get glue on your fingertips.
Possibly using small blobs of BluTac to hold them while you put glue around the edges and stick them in place might help.
Sometimes it helps to sketch the camo patterns onto the wings, fuselage and tail surfaces with a soft pencil to give a guide for painting.
Have fun, I used to love building kits, don’t have space for them any more. It’s possible to get a bit obsessed with getting details right, so be warned! 😉