I learned to use the rudder by flying the pattern with the rudder and elevator only, didn't move the ailerons at all. This will help to get your left thumb used to the motions required. Of course this really only works with a trainer or a plane with lots of dihedral. Be warned! excessive rudder combined with excessive elevator can lead to a nasty snap roll depending on the plane. I also found that leading the ailerons with the rudder first smoothed out the turns that the plane made.