Paint the Trim First, Then the Ceilings and Walls
When painting the trim, you don't have to be neat. Just concentrate on getting a smooth finish on the wood. Don't worry if the trim paint gets slopped onto the walls. You'll cover it later when painting walls. Once the trim is completely painted and dry (at least 24 hours), tape it off (using an 'easy release' painter's tape), then paint the ceiling, then continue on to wall painting.
To Avoid Lap Marks when Wall Painting
To maintain a wet edge, paint an entire wall all at once. Start at one end, running the roller up and down the full height of the wall, moving over slightly with each stroke. Move backward where necessary to even out thick spots or runs. Don't let the roller become nearly dry; reload it often so that it's always at least half loaded. Keep the open side of the roller frame facing the area that's already painted. That puts less pressure on the open side of the roller, so you're less likely to leave paint ridges. Looking to paint your doors next, these are the tips you need to know.