You can download the pdf file below and practice these exercises about 10 minutes/day to develop that technique. If you work on it every day, it should take you a few weeks/months to get decent lip trills.
A few tips:
-Play the bottom note slightly sharper to bring it closer to the upper note. It'll make it easier to move from one note to the other.
-Try to relax. It'll help you keep a good air flow.
- Don't forget to make a nice sound when you trill. Tone quality always counts. Even during lip trills!
-Trills in the high register are more difficult but practicing them will make you stronger and you'll find it easier to play the lower ones afterwards.