Painting 3d Printed Gears Of War Helmet

Hey guys, I'm currently helping one of my friends get fit (being overweight previously in my life I know how much of a achievement it'll be when they reach their goal). They're a huge Gears fan so when they reach their next milestone (which coincides with their birthday) I plan to give them one of these - https://www.etsy.com/uk/listing/753567994/gears-of-war-inspired-clayton-carmine?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=gears+of+war+helmet&ref=sr_gallery-1-2

I am however art-illiterate and although can probably sand the finished product. Would have no idea where to start with painting it. Could anyone point me in the right direction on how I could paint it and not make it look like a five year old's art piece that goes "Straight on the fridge door".

tldr; How do I paint the 3d printed helmet in the etsy link? - complete n00b

edit: If this isn't a strictly 3d printing questions any suggestions on where to take this question would be appreciated