If you are concerned about messing up, practice on a few scrap pieces first! We all start somewhere.
Personally, I wouldn't spend the money on spray adhesive. Just put a very light coat of resin on the part you want to glass, place a piece of fiberglass onto the resin (it will stick) then gently brush more resin on top of it. I usually 'poke' the piece of fiberglass with the bristles of my brush in order to help the resin penetrate it and work out any bubbles. You don't need a lot of resin to saturate the pieces, any excess resin will be added weight with minimal strength benefit.
Another tip, work in small, symmetrical areas. Fiberglass impregnated with resin will change shape ever so slightly as it cures, and it would be a lot of work to try to do a helmet in one go. So plan ahead before you start mixing your resin. Cut out pieces of fiberglass, as the gentlemen above recommended. Bigger pieces are better, but fiberglass also doesn't like sharp corners, so take that into account when you are getting ready.
So once you have your fiberglass pieces cut (I would just cut out enough to do one section of a piece at a time. Say enough to do the visor portion of a helmet. dry fiberglass is very light, and pieces can get blown around, especially if you are working outside) mix up your resin. I would work with an ounce or so of resin at a time, so you won't be rushing to use it all up before it starts to gel and cure. Some sections may require a little bit more, but that's up to you to figure out!
Also, to answer another question you posted, I always try to coat the inside of a part with a layer of resin as well to saturate the cardboard model. Once that is cured, I begin to fiberglass. The reason for this: on my first pep model, there was some warping. So naturally, I tried to sand down some of the higher areas to even things out. Unfortunately, I didn't coat the inside of the model before using the rondo method, so I ended up sanding into parts of the cardstock, which required me to scrape out the cardstock in some areas and spend time filling those holes with bondo.
So my personal favourite order of operations:
-make pep model from cardstock
-reinforce inside of model with Popsicle sticks, cardboard, etc. (Gives rigidity and prevents warping)
-coat outside of model with resin, doing it in small sections, not all at once, will help minimize warping.
-remove supports and coat inside of model with resin
-add a few layers of fiberglass to inside of model
-sand and bondo outside of model as needed. I say as needed, because not all areas of a model necessarily need to be hit with bondo. A lot of the sharp edges just need a light sanding and maybe some spot putty along the corners drying the final stages of finishing.
