What I did on my first hack was to connect the wires from the mouse hack to the trackball using twist on wire nuts. I then plugged the mouse into my PC. I tested the trackball movements, then swapped the wires around until it worked right.
I then soldered and heat shrinked the connections.
Just be careful messing with the wires while its plugged into the PC, the best way to do it is unplug it first, but this can be time consuming for a PS/2 plug, cause you might have to reboot the PC each time. But taking the time is better than shorting out your PC.