I can tell you how I made the helmet in my little Avatar photo.
(I couldn't post a photo in the gallery because all my shots have people in the background...I don't have a good one of the cake alone)
The helmet is small, but could be adapted.
I made the face mask 4 days before it was to be put on the cake. It's made of gumpaste and fondant 50/50. I used a clay gun (or extruder) to get the 3 long coils + 1 extra coil (to cut up) to join the long ones together. I basically just left the 3 longs for a few hours on parchment paper to dry in the shape of a U, (they were separated from each other), and the extra (also separated) I just let dry straight.
After a few hours, I built the mask...I used a large sized Crisco can (on its side) and put my 3 U's on, then I cut pieces from the straight coil (I also cut the long way to get a flatter back) to join the 3 U's together in four places (2 in front, and 1 on each side). I used Elmer's glue to attach because I couldn't get them to stick so well with water, and I knew it wouldn't be eaten. Once the mask was constructed, I left it on the Crisco can (w/ a thick magazine on each side to keep it from rolling), until I needed it.
It was rock hard, but I was able to easily cut some off of each side because it ended up sticking out too far from the cake, I thought.
For specific team colors and logo, I looked on-line.
I printed out the GATORS logo (about 4"), cut it out, and used that as my template. I rolled blue fondant thin, placed the template on top, and carefully cut around. I made one for each side of the helmet and I did that the day before the party.
For the cake, which was 4 alternating layers of chocolate and white WASC, I used 3 - 6" layers + 1/2 of the Sports Ball Pan (Wilton), also 6" across. I stacked with buttercream in between each layer.
Once stacked, I carved out the space behind the mask. There's no real advice for this...you just have to eyeball it, and cut a little at a time.
Once I was happy with the shape, I used ganache made from a large Symphony milk chocolate bar + 1/4 cup of heavy cream (read that tip here on CC) to ice the outside of the cake. I chilled it and smoothed with boiling water and a spatula. Loved this and can't wait to try it using a Cadbury Dairy Milk choc. bar (my absolute fave). The thing I would do differently is to make a double batch of ganache. I had enough to cover, but I skimped on the part where I carved, instead focusing on the dome which I wanted very smooth for the fondant that was to come.
After the initial orange fondant was on, I carefully transferred the cake to the board and added the finishing touches. Lastly, I placed the mask (which I had already sized) up against the cake...it's not attached really, but looks like it is.
By the way, the board is all fondant, I made it 2 days before the party...I used the extruder to make all the lines and numbers. The opposite end is also blue, and has FLORIDA spelled out. Here's the one I was trying to copy : http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1199214
One final thing, I pushed a single bubble tea straw through the cake, from top to bottom before ganaching. I didn't use any cakeboards in between layers, and I'm pretty sure I didn't need the straw, but I had just gotten a 50-pk. and wanted to try them for the first time.