well for the inner strips i just cut from the sloppy ends and enough length for added strength. But for the outer ones, measure, measure, measure, (I actually made 3 mark and with a ruler lined them up to make sure the lines came out straight), and then as mentioned above, score with an xacto knife. I usually do one light pass and then add more pressure, and then eventually you will be able to snap them easily. Once you get the hang of it it will seem more redundant than overwhelming. But in the end you'll appreciate how good it will look.
PS. Check out the tutorial page for info on the specific sizes of the strips. The thigh and shin strips are different than the forearms and biceps.