Make sure to cut all the way through the caulking to avoid damaging the paint when you pry off the trim.
To protect the walls, hold a scrap of wood against the wall and rest the back of the pry bar against it as you work. There’s not really a way to protect the jamb besides being very careful when leveraging the trim.
You can always measure out the space around the door with a tape measure if the other options aren’t possible. Measure out the length and width of the trim space on all sides of the door to find out how much trim you need. Narrower trim can be a good choice for simplicity and appearance, but make sure you leave enough material to cover the gap between the door frame and wall. Wider trim stands out more, but it’ll cover more of the wall. Make sure you have enough space for it.
It’s okay to leave the trim a little long at first. You can adjust this as needed later. It’s helpful to practice using the miter saw on scrap wood first. You are usually able to adjust the saw more than you expect to get a better-fitting joint.
Avoid placing the nails close to the ends of the trim. Depending on the thickness of your trim, you may need larger nails. Hold the nails up to the trim to see if they’re long enough to go all the way through to the wall.
Hold a piece of cardboard against the wall to prevent your tools from damaging it.
You’ll need to let the caulk dry before painting or staining. Check the label information to find out how long this takes.
Use an oil or latex-based primer before painting over wood.
