You take some kitchen shears or poultry shears and remove the backbone like this:
It's called "spatchcock"...yes... really it is
Then press down to flatten (squish) them a bit... season with a rub or whatever you want.. and toss them or it on your grill... this is on my kamado and is indirect heat even though it doesn't look like it. So I recommend pretty much indirect heat (if you're using charcoal, bank it all to one side and toss your chicken on the other.. or if gas, turn off one side and put the chicken on the side you turned off... using a drip pan or folded foil underneath is a good idea).
Probably take a bit over an hour to cook... I'd check at an hour to see how they're doing.