What Miamirick said
3 hours on the grate
2 hours wrapped in foil with a little apple juice or cider or pineapple juice even beer or something like Dr. Pepper (it's ok to get crazy with the liquid)
1 hour unwrapped again and directly on the grate to firm things up
Note: these times are pretty close however, you may find that with practice you like to leave them in the foil a little longer or even a little less. This may or may not add additional time to the other steps.
I recommend starting with the 3-2-1 then if they are too fall apart tender you can reduce the time in the foil to say 1.5 hours while increasing the initial time on the grate by 30 minutes. That last hour is intended to firm the ribs back up a bit and help create the bark on the outside. The time in the foil tends to really tenderize the meat and will also soften the outside of the ribs to the point that there is no bark at all. I highly recommend at least 20-30 minutes on the grate no matter how long you leave them in the foil.
A few minutes of meaty side down on a hot grill may also work to firm the ribs and create some nice bark.
And just like MiamiRick said... for baby backs, just reduce the initial time on the grate by 1 hour.