ok yea i understand the breaking down part but i dont understand what to do afterwards
Ok. Every bullet has four components: cases, powder, lead, and primer. Each single round requires one case, an appropriate primer, and an amount of the specific type of powder and a variable amount of lead. These components can be bought as certain merchants but it's easier to break down ammo you don't need and use the parts to refill cases for ammo you do use. And you'll automatically get cases as you use weapons. Each round fired has a percentage chance of leaving you an empty case you can refill. The most important thing to remember is that there are specific types of powder and specific primers and and they can only be used for specific types of ammo. For example, breaking down 9mm ammo gives you small pistol primers and pistol powder. You can use the powder for other pistol ammo and primers for small pistols. The refilling table should give you the formulas you need.
Typically, I don't bother doing the math. I just break down a bunch of ammo I don't need and use it to make as much ammo I do need.
And don't worry if you don't get the hang of it. It's not really all that essential. With the exception of a couple of really exotic ammo types, bullets are cheap and plentiful. You don't have to press your own.