How to Create Ammo
First of all you have to be in one of those towns where you can do that (repair icons on the sector, I think), like in the town of Ernie.
Then you go into Operations, scroll down and select craft ammo.
Pick who you want to craft the ammo with.
Then you just pick the ammo you want to craft and off you go.
(The cost is listed when you hover over the item. Like for example, 7.62 Nato Match will cost you 10 parts and 30 Nato standard ammo, and will give you 30 Nato match ammo).