Been there, done that...the following is what works for me:
First, be sure there's plenty of propane in the tank. If there's no meter, throw some very cold water on the tank - the condensation will form at the level of the propane inside the tank. If you're under half a tank, you may want to have another tank at the ready.
I haven't had any success with direct grilling and ribs - everything I've tried winds up "black and blue" (burnt on the outside, raw on the inside). Think roasting - low, indirect heat on a closed grill.
Get some woodchips (hickory, pecan, or whatever you plan to use) and soak 'em in water for at least an hour. Remove the cooking grates from the BBQ, fire up the gasser on full high heat (all burners) and let it pre-heat for about 20 minutes with the lid closed. While it's pre-heating, drain the wood chips, wrap 'em in a foil packet or two, and poke some holes in the top of each packet. At 20 minutes, throw the packet into the gasser on the left side - on top of the lava rocks or the burner furthest to the left, then close the lid. When you see the first wisps of smoke leaking out from the closed unit, turn off the right-hand burner, turn the remaining burners (including the one under the wood chip packet) down to medium-low or low, put the cooking grates back in the BBQ, then put in your ribs on the on the right-hand side and close the lid. Mop 'em with apple juice at about 45 minutes. Figure they'll be done in 90 minutes to two hours. You can sauce 'em during the last 20 minutes.
It won't be like ribs off the smoker, but it'll be good enough to fool those who don't know any better.
Good luck, and let us know how it goes.