I glassed the inside of my (cast resin) helmet, it's not super difficult.
Wear long sleeves, tie back long hair, wear gloves, goggles, and a respirator.
The key is to prepare well. Pre cut your fiberglass, choreograph the order you want to work (I started in the dome and worked down) and lay your pieces in the correct order, first piece to go in on top of the pile.
Best to do this on a day when you can have fresh air and 55-75 degree temps - on cold days the resin cures slooooow and hot days it's a struggle to get it mixed and lay down the wetting coat before it starts to kick.
Wear your respirator. Not a dust mask, an actual respirator with face seal and filters.
Pre-measure your resin, it might be a good idea to have two small batches worth measured out and ready to mix. Work your resin in small batches, and use a wide shallow pan lined with foil (fresh liner between batches) to hold it while you work. Tall narrow containers will concentrate the mass and make it cure way faster. You can always mix more and if it is starting to kick, stop working and let the current batch in the helmet finish curing. Trying to 'push' it will only end up making trouble. Don't bother trying to save brushes between batches of resin, get a bunch of cheap cheap chip brushes, use a fresh one each time and toss them when the batch is done.
Wet down the surface you are working with some resin, carefully lay down your glass so it sticks in place(I used woven cloth, chopped mat could also work). Dip your brush in resin and use a stippling motion to sink the glass into the resin - a brushing motion will slide the glass around on the surface of the wetting coat of resin, it will not help. The resin does not add any strength beyond holding the glass in place, and sticking layers of glass together. A heavier application of resin only adds weight, so use the least amount you possibly can. The goal is to 'wet' it out until it's basically made clear by the resin, using the least amount possible.
The glass texture should still be visible (promotes adhesion on the next layer), but it should all be the color of the resin. If there's any part of the glass that's still the glass' natural color, there's a void between the helmet and the glass, so press until all the glass color is gone. A little buildup of resin on the surface is ok, but remember - least amount possible.
Overlap your pieces slightly as you lay them down (they should be sized and cut to do so). I did the dome in one pass, and then did the sides, and followed up by running some woven glass tape (leftover from making boats, you can get it online) at the edges and around the brow band to finish and reinforce.
I only did one layer in my
cast resin bucket; for a print a few more layers is a good idea. Each layer you should let cure, sand lightly, remove dust with air and a tack cloth, and repeat the wet, stick, stipple. Use the cure time to cut new glass and pre-measure the resin parts. Offset seams between pieces in each new layer, this will help build a more consistent thickness and avoid creating lines of weakness. If using woven glass, I suggest rotating the grain 45 degrees between layers to improve strength in all directions.
On the last layer it's probably ok to add a little bit more resin on top of the glass to give a smoother surface texture when you do the final sanding.
Wear long sleeves, tie back long hair, wear gloves, goggles, and a respirator.