Okay, lemme see if I can put my technique into some kind of linear explanation without rambling too much
First I'll say that getting the helmet shaped up properly is really the most important part. Attention to the shape of the fin, depth and consistent thickness of the eyes, mouth and air vent openings is really a subtle but important detail. If those are off, no paint job will look great.
So, that out of the way - the main materials I use are Testor's primer, Testor's gold as a base coat and Antique Gold Rub'n'Buff and some Testor's Gloss Cote for the final step.
For the weathering I currently use mostly acrylic paints that I mix to various colors - nothing specific really, I just eyeball them. I typically use white, black, metallic gold an aquamarine type blue and maybe a bit of a rust or brown as needed.
I used to experiment with enamels for weathering but it was too messy and hard to work with IMHO. Personally I like the results better with the acrylics and you can clean up with water.
I typically just primer the entire helmet and then put a base coat of the Testor's gold. After that I put down a full coat of the Rub'n'Buff. All of that is the easy part to explain.
The weathering starts after I get the Rub'n'Buff on the helmet. I mix up some black and brown paint with some white for opacity until I get a nice dark muddy color and thin that down with water until it's not much more than a wash. I dab the wash on the helmet and let it dry... this is really where it starts to get into the whole "by eye" part where I really have trouble narrowing down my technique.
From that point I start dabbing on the paint a bit heavier. I dab it on and wipe it off and then go back and add touch ups of Rub'n'Buff. I basically just work back and forth until I get the mottled look I'm going for.
After I get a good bit of the brown washes, I add some more white and greenish blue to the mixture until I get kind of a cool mushroom color and then apply that in spots to give it some hints of a patina.
Once it's all dry, it usually doesn't look all that spectacular and this is where the final step comes in. Folks tell you not to clear coat Rub'n'Buff because it dulls the finish. But, Rub'n'Buff is too shiny by itself - it NEEDS to be dulled down. The Testor's Gloss Cote is really more of a semi-gloss finish. I like it because it really does two things - it unifies the painted surface so it doesn't look like a mixed media mess and it dulls the Rub'n'Buff's overly brassy shine.
The original helmet I had for a while wasn't quite as glossy as the semi-gloss but it was more glossy than you can get with a dull coat. I think some of that is because it was 20 years old. It looked a bit glossier in the movie so erring on that side is preferable in my opinion.
I think that covers everything... hope that explanation is intelligible. I've tried video-recording my work but it doesn't fit well into my rather chaotic, cat-like workflow and unpredictable attention span.