Just like any other speedy chop you try to pull, you need to start slow and work on symmetry and evenness. Whereas with hands people seem to understand they need to concentrate on sticking and phrasing, they will plop their feet on the double pedals and just assume nervous energy will carry them through.
Start off with slow single alternating strokes, real slow, get them even and as close to sounding the same as you can. Do this with a metronome if possible. Gradually (over the course of days, not minutes) increase the speed. Towards the end of the exercise you can add in hands if you feel comfortable enough. Later as you build speed and precision, you can start transferring rudiments to your feet, again concentrating on "sticking" (footing?)
Just as you can't do everything on the drums when you first get on them, double pedal work takes time and practice. It is deceptively difficult, because how hard should budda-budda-budda be? So start out slow and practice, practice, practice!