Roller nap matches the wall
3/8" nap for smooth drywall, 1/2" for light texture, 3/4" for stucco and heavy texture. The wrong nap is the difference between smooth and orange-peel — and it costs four dollars to get right. Cheap roller covers shed fibers into the finish; buy the mid-tier cover and one spare per room.
The cut-in brush is the job
An angled 2.5" sash brush does every edge in the room. The technique: load the bottom third of the bristles, drag (do not push) a 3-inch band, and let the angle do the corner. This is the single skill that separates DIY from pro-looking walls — ten minutes of practice on a closet door is the best training investment in painting.
Tape, light, and the last 5%
Blue tape seals better when the top edge is burnished with a putty knife; remove it while paint is wet for the cleanest line. Inspect each wall with raking side-light before the second coat — touch-ups blend invisibly then, and never afterward. Professionalism is mostly inspection with better light.