Modeled in blender starting from an STL rip from Destiny the game.

The entire helmet as a single part is in the zip, because it is about 100mb (lots of smooth curves).

Printing Prep

It is to scale in game, which is probably big for most people.

To scale to your head, use something like meshmixer and cut off the lower neck portion. Uniform scale until your head just barely gets through the neck comfortably, then use that scale for the rest of the helmet (the neck is the tightest portion).

Once the helmet is to scale just slice it into parts big enough for your printer and you're good to go. 

Suggested print settings

Use only as much infill as you need to support top surfaces, so like 5-10% with 3-4 top layers, becuase otherwise the helmet will be very heavy. 

The ears are fragile so I suggest printing it in PETg.

Painting and Finishing

For finishing the helmet cover the entire thing in BONDO then sand smooth with 300 grit. If parts misaligned while gluing use 150 grit and lots of BONDO to smooth it out. Finish the sanding step with a coat of filler primer, then fill any areas that still look bad with more BONDO and re-sand.

For paint, use any matte paint of your choice. If you want to keep accurate to in-game, the entire helmetis a single color, except the two little latches at the bottom. 

For detailing, use either a low grit sandpaper (80) and lightly drag it on the hard edges to reveal the grey primer, or drybrush silver (I am bad at dry brushing so I used the low grit sandpaper technique). Then do a wash of watered down black and brown paints to make the small details stand out.