When I started out, I stared with 2 chords, A and D. Once I could make them with all the notes sounding, I would switch back and forth at the fastest pace that I could do with a metronome for about 1 minute. When I could manage the change at 60 times a minute, I added the E chord. Then I would do A to D, D to E then E to A. To this day if I find a new chord change that is giving me trouble I go back to this routine.