This is hard to do with a popcorn ceiling,I use an edger,or you need to push the tape a little ****er up with like a putty knife or even put the tape a little ****her on to the other surface,or alot of times you just got to get a nice small quality brush (purdy wooster) come to mind like a 1 inch or even a half inch brush and cut it in by hand taking your time.
Some people end up putting borders up after they attemt to do this.

Popcorn ceilings are nearly impossible to cut in straight. I usually paint both the edge of the ceiling and wall, then repaint the walls later. Use as much paint on each stroke as possible.