Double check to make sure everything is turned off! Sounds like a no-brainer, but it you don't want a big mess to clean up.
Holding the ball valve against the pipe, make marks on the pipe with a marker so you know where the pipe needs to be cut. Make sure you don't cut off too much, as part of the pipe should sit inside the ball valve. It's always easier to cut too little instead of too much!
Put towels underneath the pipe before cutting the pipe. With your pipe cutters, slip them over the pipe, tighten and rotate them around until the pipe is cut.
Follow proper instructions for installing the particular type of ball valve you have. Once installed, make sure you test it at a low pressure.