The next step isn't entirely set in stone. Some people just mix up some rondo and swish it around, then repeat until they get the desired thickness, but rondo alone can be fairly brittle and won't have much flex to it, which makes it better for props. If you want better durability and a bit more flexibility, you'll want to go with fiberglass cloth or mat. Spread some resin, lay down the fiberglass, let it cure, then do it again until you get the desired thickness. If you do cloth, to my understanding it's better to do as large of an area with one piece as possible to reduce the number of seams (weak spots), whereas with the fiberglass mat, because it's somewhat loosely packed fibers (as opposed to the woven cloth) you can just rip pieces off and lay them down bit by bit to cover the inside. In either case once you have enough layers built up (2 minimum, some go even thicker) you'll want to sand the inside and then do rondo to get it nice and smooth (resined fiberglass can be very sharp, and you don't want to essentially shove your head into a big bowl of it).
After that step comes bondo and sanding on the outside to get things nice and smooth (expect to repeat the bondo-sand process a few times if you want the smoothest results with no pockmarks or dimples). Then comes primer, which often will reveal if there's any spots that need more bondo and/or sanding (another step that may require repeating a few times) before it's ready for paint.
And if you need to cut the helmet into sections to get it to fit, so that after the fiberglass phase. I know it would be easier to cut in the paper or even the resin stage, but it's more likely to warp then because it won't have the added support to keep its shape.