How do you paint a clean corner?

Have you tried painter’s tape? Tape the side of the corner on the wall you’re not painting, paint the other wall, wait for the paint to dry then reverse. Painter’s tape is a wonderful invention.

In the UK you can buy a special plastic ttachment that fits on to your brush to help with straight lines, not sure what its called though SORRY! I suggest you get a professional in! I’ll try to google the too for you later :)

There’s really no need for a professional, @sakura. Just go buy a roll of this (I think it even comes in 2” size, so you can be really messy), and paint away. No muss, no fuss.

yes but I am lazy!!

You can hold a piece of cardboard in the corner at an angle; first this way / and then this way \.