The trim of windows and doors can be the hardest part of a home painting project, especially if you haven't wielded a brush before. Rolling is the fun part, but the precision needed to make trim look great takes a bit more preparation and practice. There are two things you can do to make the process flow. First, invest in good masking tape. The better stuff won't leak like cheap tape, so you keep crisp edges. Second, get a good brush. For a few more bucks, you get a much better tool, and that can make a big difference in how the job goes.
Tape off the wall areas around the trim. Rub over the tape with the rag. This seals the tape well to the wall. With the tape in place protecting the wall, leaving the trim exposed, you don't have to fret as much about mistakes.
Pour some paint in a cup. A lightweight plastic cup is fine. It should be wide enough so the brush fits easily. It's a lot easier to paint trim using a cup to carry the paint, instead of a heavy can.
Start lightly. Just dip the brush bristles in a little. Less than 1/2 inch is fine to start. Apply the paint on the trim in light brushstrokes. Make the strokes along the length of the trim; don't try to paint across it horizontally. Use the full width of the brush, provided the trim is at least as wide as
Load your brush more heavily as you become comfortable. Dipping the bristles just under an inch into the paint is about right. You should be able to pull the brush and cover six to nine inches of trim without drips. If you can't, load a bit more paint. If the paint drips, load a bit less.
Keep an eye on the area of trim you last painted. Sometimes drips form a few seconds after you finish the brushstroke. If you see them, a swift, light downward sweep will eliminate them.
Remove the tape when you have completed the trim.