Not gonna lie man. Helmets are by far the hardest thing to do in leather, at least for me. It took me ten years before I found a pattern I could make look like something not crap and be comfortable on the field. I hate to be a downer, but really you should cut your teeth on some more basic stuff: arms, shoulders. After that try some gauntlets and gorgets. Then start playing around with wet molding by making some masks or cops. Then try your helmet.
That helmet you posted is essentially just an open-faced barbute:
The one you posted is also made of plastic, not metal.
Something like that is particularly hard to do in leather because leather's going to have to have some seams somewhere. Older types of helmets, like spangenhelms, are much easier to make in leather, as are helms like great helms that don't have much dishing, just curved pieces. But with all the seams/lines/decorative whatevers on that helmet, you might be able to do it by sewing a bunch of smaller strips together. Tedious, but it might work.
Sorry I can't be more help. Helmets is hard...