Location: Outside Walkway - Above the tunnel in the back of the walkway
Before hopping over the gap to the side along the walkway where the TIE fighters swoop by, if you want to see a hidden helmet easter egg, then use force grip to stack up a few crates next to the rooftop of the tunnel at the end of the first walkway then leap up to the rooftop of that tunnel. It is best to stack a cylindrical crate on its side (standing up) then stack another cylindrical crate on top of it or stack two rectangular crates on top of the cylindrical crate. Position them with the left analog then set them down lightly by using the right analog to lower them before letting go. Either way, make sure the crates are stacked right below the rooftop then hop on top of them and double jump to the rooftop. Use the force dash (L1) to help gain some extra air time during the double jump if needed. It is best to attempt this on the easiest mode since the TIE fighters will be swooping by and firing at Starkiller the entire time from the left side.
The helmet will be at the end of the rooftop. It can't be collected. Stare in amazement then drop down and get back to the situation at hand.
If you're out of crates, you can also grab the nearest column (steel girder) with Force Grip and stretch it toward the walkway. Jump on top of the steel column then double jump and force dash onto the rooftop. Sometimes the column can be hard to bend properly in order to get on top of it though.
Another way to see this is to jump off the right side of the walkway that leads up to the AT-ST boss - the long piece at the beginning of that walkway. After blasting through the door, shortly after riding the elevator up, move to the far left edge of the right walkway then perform an extremely extended double jump and air force dash while moving toward the rooftop of the tunnel on the walkway below and you'll land on the rooftop and be able to see the helmet. Please note that this jump has to be very extended and you will have to press the jump button (for the double jump) and force dash button only after you have traveled a great distance in the air in order to maintain greater air time while moving toward the rooftop. You can also jump to the steel girder to the right of the rooftop then perform another double jump and dash over to the rooftop from the girder. The steel girder is easier to jump to, but you'll have to perform two sets of double jumps and dashes altogether.