Before setting off for adventure, be sure to equip yourself for the journey ahead.
Heat Gun (or other SAFE heat source)
Stencils - I got a few from this costume research site
that worked quite well for the cheek guard and that crest up front.
2 Matching Tankards - Since it's hard to clean these out once they're on the helmet, try to find a pair that will fit the whole drink
can inside. If not, I'd suggest finding some half-sized drink cans or water bottles to keep inside.
Craft Foam - Sheets both 1/4" and 1/8" thick, They are sold in craft stores but for larger sheets you may have to go online.
Start by stripping your helmet of its cup holders and vizor. Any way goes so long as the rest of the helmet remains in one piece. I wound up using pliers and a small handsaw for this.