To make it simpler, think helmet is a sphere.
Now I need to place a star or any other complex object you can think of - on it. It's a big star, and it looks like taking the half of the helmet - so it should be spherically bent too. It can be separate object.
(P.S. I watched another tutorial where the guy used boolean modifier in order to intersect two objects. The booleans seem to work on simple object. It didn't work in my case.)