To craft ammo you will need to find the “Reloading Bench” and know the “recipes” to craft each type of ammo; have “gunpowder” and the “box of bullets shells”.
3 magazines with the recipes: Handgun bullets, Shotgun shells and rifle cartridges.
You will need to find a “box of bullets shells” that with be depleted every time used and you can craft any type of ammo with it (can be found at gun-stores and other places and weighs 5). (It could be also one box of each type of ammo: Handgun, Shotgun and rifle).
You can obtain gunpowder from any type of ammo to craft the one you need also is already found at gun-stores.
The “reloading bench” can be randomly found at hunting cabins or garages. (Extremely rare).
On late game you may get stuck with a bunch of ammo that you don’t want, why not transform it in the ammunition of the weapon you like to use; maybe you like handguns or shotguns more.