As Testkil said, there you can find all the files you will need. You make it the same way as the skin, however, keep in mind that you can make the helmet transparent if you wish. Most tips that would apply to making the main skin, also apply to making helmet skins. Remember the light two-pixel area around sections is the 'bleed area' and needs to be filled in with the color you want (if you don't want weird colored lines on the edges of the helm).
If you want to use some transparency, to (for example) have the eyes show through on a helmet or visor, I often will put Dave's face under it to give me a rough idea where the eyes should be. The helmet is bigger than the head, so I mean ROUGH idea. Usually after testing in game (I test in Singleplayer) I have to adjust it. So there is some inevitable trouble shooting there if you want to do that.
Otherwise make it like a normal skin, but keep in mind the size and shape. There's a shading overlay that you can use for the helm, just like there is a different shading overlay for the skin -it will help shade things to look nicer!