In my experience...
I start by feeding juveniles roach nymphs when they are big enough to eat them... If you get them to get used to the roaches when they are young usually they will eat them when they get older. Each chameleon is different... I have a few that LOVE LOVE LOVE them some Dubia, however I have a few that are scared and will even fire up/run away from a roach.
Good Luck,
Todd