The first thing you are going to need to do is hook one of the internal game functions if you want to render over it. When I say hook, I mean intercept a function and insert a jump to your own function, and execute that. Once you have hooked a function such as Present(), you will be able to draw over the game. You need to use tools such as cheat engine and ollydbg to find different memory addresses which accomplish different things, such as the value for ammunition in memory. Hacking simply consists of modifying these memory locations.