How do you put the convertible boot cover on the top once its down on a 1994 cutlass supreme 3.4 Liter

I have that same car. I'm presuming you have the correct boot cover, and presuming none of the snaps or velcro strips are missing, and presuming the roof is all the way down. (ie. Resting on the stops. Sometimes, if things are stiff or cold it doesn't quite go all the way down. Simply push it down the final couple of inches by hand before trying to put the boot cover on.) With that said, it should be fairly self evident how it fits, and if you have the supplementary manual that comes with convertibles, it is all in there. Lay the boot cover loosely over the folded top.There is a velcro strip along the front of it that attaches to a velcro strip on the car located just behind the rear seats. There are two or 3 metal snaps on the two front corners of the cover that attach to metal snaps that are attached to the car. If these are missing, you will need to have an upholstery shop replace them before you can use the cover or it will blow off while driving. The back edges have stiff plastic flaps which tuck between the body and the folded roof. Voila... you're done and the car looks even more awesome than it does already. If the cover is like mine, the vinyl has shrunk a little with age and its hard to get the rear flaps to stay in place. The cover "floats' a little while driving which I find a little annoying. I haven't been able to figure out a fix for this other than having a new cover made at an upholstery shop, which I may do some day. Good luck. Hope you're enjoying your car as much as I'm enjoying mine.