The front portion is held up by a couple of plastic shafts with metal clips. These fit into square recesses in the metal reinforcements on the underside of the roof. The rear portion is held up in two ways. One is by plastic tabs which extend past the edge of the assembly and rest on something (presumably metal) above the headliner. It is also held up by a rectangular piece of a velcro-like material which engages a similar material on the underside of the inside of the roof assembly. This stuff is probably to prevent looseness and vibration.

As far as how to tighten it up, it depends on whether the front portion or rear portion is more loose. The rear portion might be tightened up by simply pressing up firmly on it. The front portion may be loose if it has been previously removed (say for the sunroof rattle fix). It's typical for the metal clips covering the plastic shafts to get bent when the assembly is removed. If those clips aren't bent back correctly the whole thing could stay loose.

I did the homelink retrofit (just bought the part which includes the sunroof switch, lights, homelink, and sunglasses holder, and installed it) and I think there might have been pictures and instructions here on the forum somewhere. Not sure, though.

Hope this helps.