The difference between an amateur and professional-looking paint job is almost entirely in the prep, not the painting itself.
Clean and patch first
Wipe down walls, fill holes and dents with spackle, and sand smooth once dry.
Tape with intention
Press painter's tape down firmly along every edge — this single step prevents most bleed-through.
Prime where it matters
Stains, patched areas and color changes over dark walls all need a primer coat first.
Only after all of this should the roller come out — and it shows.