Basically, I just follow the kit instructions and bung the whole kit together before painting. If you want to show the interior, then you're going to have to leave off the upper hull until after you've gone through and both made and painted the interior. That done, slap on the top, mask off the open bits and go straight to painting. Because AFVs don't have cockpits, undercarriage etc like aircraft, they are very easy to make, and most of the real skill comes into play in super-detailing, painting and weathering. The only items I leave off AFVs until after painting are external machine guns and aerials, which tend to end up snapping or flying off into the depths of the carpet. Additional baggage etc can all be stuck on before painting or left off until afterwards, it's very much a matter of choice.