Watch the Lesson 6: Popping Technique online guitar lesson by Stu Hamm from Funk Bass for Beginners
Just like with thumb slaps, with finger popping it's essential to understand both what TO DO and what NOT TO DO, so I'll break down both for you so you'll know what to shoot for. Proper technique is critical here both to keep you from getting injured and for getting a great sound. You need to work this SLOW. Take your time and practice staying relaxed and getting the right tone out of the strings. If you do this right and you drill it enough, you'll find it becomes almost an effortless motion, which will set you up for combining pops with slaps in the future.