Hey SBJ, I will do my best to help you out here.
My method might not be the same but I will explain based on what you said in your post.
1) for the clay sculpt you can use an oil based clay, plasticine or similar. Oil based clay will not shrink and you can reuse it.
2) Once you are satisfied with your sculpt and want to make your mold you can use many types of silicone to do this. Silicone does get pricey though. So depending on your budget there are many options. An RTV silicone would be good because it is more rigid than some other silicones. An easy method using Smooth-On brand silicone will net you good results but you have to pay $$$. But that being said, it makes your job easier because the mixing ratios are easier than other types of silicones. There is also another way cheaper method of using 100% silicone you get at Home Depot or other hardware store. The kind that come in tubes. You mix that with corn starch and mineral spirits. It smells awful and you will need a mask when using. Depending on your ratio it will dry a lot faster than others. ie; most silicones take 10+ hours to cure. Smooth-on cures faster and so does the mixture you make at home.
3) Once you have your silicone cast on the helmet you will need to fortify the silicone with a shell. The silicone will be flimsy and worthless once you pull it off your helmet sculpt unless you make an outer 'case' that will keep the shape of the helmet. You can go to a hobby store or Michaels or Joannes for plaster stips or gauze. Dip those in water and lay them over the silicone in criss cross layers. Once they have hardened you can remove your 'case' and then silicone. Voila!
4) Now that you have your casting of the helmet you sculpted you can make the positive copy you plan on using. Go to an auto supply store and get yourself some fiberglass mat and fiberglass resin. The mat you lay inside the silicone to cover the surface of the helmet mold you made. Then mix the appropriate ratio of resin and catalyst. Brush the resin mix into the fibers of the fiberglass mat you laid. Make sure you coat it very well and get all the little nooks and crannies.
5) Now that the fiberglass is dry you can pull it out of the mold. Now you should have a positive copy of your helmet! BUT! You will need to fix the outside of it. There will be pockets of air and places you can see the fibers. You want a nicer finish maybe. You can always mess up the new layer you are about to lay just to show damage. This new layer to the outside is Bondo. You may have read about that a lot on here. Also purchased at an auto parts store. The directions for mixing will be on the bundle you get. It is like a putty you smooth out over the outside of the helmet. This should fill in any holes or disfigured parts of the helmet.
6) Sand sand sand. Fall in love with different types of sand paper. You will be using lots of it to get the finished smoothness you desire on the helmet. Depending on how sloppy you laid out the bondo will make a difference of how long you will be sanding.
7) I won't go on about painting and finishing the masks...
I hope this helped you out some. There are many how to videos online to watch. The other awesome people on this forum I'm sure have different methods and I know they will help you too. If you have any more questions Just post 'em.