After the drywall installation in a room, trim carpentry can start. But there are a few things to be checked for first. Check all the surfaces that will have molding applied to them, using a straightedge and carpenters square.
Drywall compound will frequently have been built up along inside and outside corner joints, and your trim molding will not lay flat there. So, you need to flatten these areas out, taking off the high spots with a Stanley Surform tool, and paring down tight areas with a chisel where the Surform won’t reach.