First of all you'll need to know the location (memory address) of each function you wish to use. And to add some complexity, each jj2 version has different addresses. The second thing is knowing what parameters to pass to the function. And finally, you need to write some code to call the function. In most cases this would be a DLL that is attached to the game by some other process. The second option is to use assembly code injection and then have this code call the requested function. Both concepts are an advanced topic and requite a certain amount of knowledge about assembly and other low-level CPU functionality.