This app was mentioned in 3 comments, with an average of 2.33 upvotes
I use Kegel Workout: Exerciser and it works really well. it's free (has ads, but you can live with it), you can make your own routines, schedule exercises and has notifications.
Im not a professional, but this app is really useful in terms of contracting-relaxing pacing and timings and has some instructions on how to activate the P.C. muscle. Enjoy your rock hard erection my brother
https://play.google.com/store/apps/details?id=com.workout.kegel
Best? Hmmm, haven't tried then all, but from the few I did, here Is the one I currently use:
K-Workout - Playstore Link