The usual way to get tight inside corners on trim is to “cope” them—to cut a profile on one part that will fit over the adjoining part. That means lots of slow, fussy work with a coping saw. Next time you have some inside corners to cope on standard 3-1/4-in. baseboards, try this method for doing the whole process on your miter saw. It works for ranch and Princeton styles. It takes a bit of practice to master the trick, but once you do, you’ll be able to achieve a perfect cope in less than 60 seconds and never grab for the coping saw again.
If you have a low fence on your miter saw, add a 1x4 (photo 1) to fully support the baseboard. Your saw has to be adjusted so it cuts perfectly square in the vertical direction. There will be wood shrapnel and your saw will be running for long periods, so wear vision and hearing protection.
Install base in the clockwise direction around a room if you’re right-handed and counterclockwise if you’re a lefty. That’ll be playing to your strong hand, and you’ll always be coping the same way joint after joint—no confusion.
Rest your elbow on the table or the saw miter adjustment arm for stability. Don’t try to get a perfect cope with the saw alone; you’ll be able to quickly clean it up with a file. And lastly, don’t take your finger off the trigger until you’re clear of the cut. The blade will plunge the instant you shut off the power and you’d wreck the cope.