Geez, you go away for a few weeks and the things you miss!
I used to have a fiberglass surfboard repair company--back in the day--and am applying some of the old techniques to my double wall top (the one that flew off the sawhorses a few months ago).
For big holes, use a rigid foam for a backer--Hobby Lobby and the like sells a variety of different products, and you can also use building foam. I haven't tried the spray in type used for window indulation, but I'm going to! BTW, higher density is better. Cut and stuff the foam in the hole and laminate over it.
For cracks, take a grinder or equivalent and carve the crack so that the surfaces are even with one another--also widen the crack a bit so the resin has something to adhere to.
For some of the structural stuff, that'll depend on where and what you're trying to do. Remember that the glass is only partially structural--it becomes stronger the more layers that are applied. Really heavy mat--the type used on a variety of RV and boat parts--is a heavy series of mats. This stuff is probably not applicable for repairs to a top. More on the mat below. So, build a piece to fit in the area you're trying to repair out of wood or foam, stuff it in there and start laminating.
Apply the resin with a throwaway brush--you can make one last a long time by cleaning with acetone and propping it standing up so any residual resin flows down toward the handle leaving the bristles workable for another round. Use the plastic sqweegee to the pull the resin over the mat, smooth out bubbles, etc. Remember, the resin is going to follow gravity and will pull the mat with it, so for applications which have a top and a bottom, do it in multiple steps--wet out and cure layers on one side, then flip the piece and do the same for the other side. You can use a small piece to wrap an edge after you have the desirable effect on the balance of the repair.
For pieces that can't be made to fit flat, cut the mat larger than you need, use a tape along an upper edge to hold it in place and wet it out with a fairly hot batch of resin. Be careful not to wet out the taped portion of the mat--it's the difference between cutting the dry mat with scissors and grinding and sanding. After it fires, pull the tape and cut any dry mat away.
After you have the requisite number of layers, apply a final coat of a resin/catalyst/sanding agent to the entire repair (gravity's not a big issue in this one) and let it fire off. Dry to the touch means it's sanding time. You sand just like wood, heavy grit to fine. Machines can do a bunch of removal in a hurry, so I tend to use them sparingly and usually only in the initial stages--doesn't take much to sand through a repair, so don't be too aggressive with the heavier grits, either.
The mat itself is usually sold in auto parts stores, but see if you can find a fiberglass supply in your area. They not only carry the basic mat (usually 8 ounce) but also carry a variety of other mat weights. The weight of the mat can be identified by the actual size of the mesh itself. 8 ounce has a bigger mesh opening that 6 ounce. For most of my repairs, I would prefer to use multiple layers of 6 ounce rather than one layer of 8 ounce--again, strength.
The fibergalss supply with also sell a variety of resin and catalizers, cheap applicator brushes, sqweegees, acetone and...
sanding agent.
The usual missing element in the sanding process is the sanding agent. It is a wax-like additive dumped in a regular resin-catalyst mix. You don't need much, and all it does is seal the resin so you can sand it without getting a semi-gooey surface. Sand with a variety papers depending on how smooth you want it--you can approach it like body work and get up to a wet 1500 grit polish if you want.
I don't sand between coats--if I have hairs sticking out, a real rough rasp will knock 'em down. I cut out air bubbles if I have any prominent ones.
The resin will eventually harden no matter how much catalyst you use--the more you use, the hotter it'll fire and the quicker it'll go off. A hot batch will turn a little brown due to the heat, so gauge your needs accordingly. You can usually get two-three layers a day with a moderate use of catalyst. Weather will affect it as well--a hot day vs. a cold day. Be smart about doing this around a garage heater or any type of flame
I'm sure I'm missing some stuff, but ask and I'll try to answer. It's pretty easy stuff to work with, so don't be afraid that you'll get in over your heads. Like Bondo, if you don't like it, grind it off and start over. Enjoy!