Don't know about jelly, I've just used the regular resin and hardener in the past. I usually get an extra tube of hardener because I usually put too much in but I figure better to go off to quick than have it not set!.
Simple mat is fine for the matting.
For the Fiero headliners I have done I mix up the resin/hardener then brush on a coat onto the board on the side that will be facing the cabin (ie where you will glue the material to), lay the glass mat on it, then brush in more resin mix into the mat, using the brush to work it into the mat. I usually only do one side with mat, one the other side I might just brush resin into the board itself and let it set, just to help strengthen it. Generally I don't worry about wrapping it around the edges of the board.
Once it's set I use an flap wheel on an angle grinder to 'gently' take out any bumps and unevenness in the mat. I might have to redo some of the matting in places. The smoothness of the surface will greatly effect the look of the finished headliner and while it does not have to be perfect, the smoother you can get it the better the finished item will look.
I get the headliner material from the local Jo-Ann fabric store.Once I've got the cabin facing side nice and smooth I glue on the headliner material using proper 3m headliner spray on adhesive. I seem to recall it comes in different 'strengths'. I always used the strongest heaviest duty stuff I could find.
Cut material to size, spray half the material and half the board. Once tacky, lay the dry half of the material on the dry side of the board and then fold over the glued half onto the glued half of the board, keeping a little tension on it and smoothing it out as you go. Once you've got that half in place, spray the other half of the material and board and just lay in place making sure to stretch it out as you go to keep it smooth again.
Make sure you leave enough extra around the edges to wrap around the edges of the board and any openings.
If you screw up the material install, I've found the best way to remove the material is to use the flap wheel on the angle grinder to gently remove it so you don't go through the fiber glass underneath. Then start again with the material (which is why I usually buy twice as much as I need!)
Anyway, that's how I've done mine in the past and so far they are holding up just fine, even in the heat of an NC summer.
Anything I might say is probably worth what you paid for it, so treat it accordingly!