1991 GMC Sonoma tracing a short, using multimeter?

Thank you, my girlfriend has a 91 mercury sable. She said having trouble starting then drained the battery. I bought a new battery, put a new voltage regulator on and cut and put a neg, battery post connector on, and the mechanical key switch, still nothing. So we decided to call someone out lol, he put a starter in still nothing. He said there was a short to the ground somewhere to my girlfriend, I was working. He quoted a outrages amount. Being a little nice. So that was the end of him.I bought a multimeter and guna learn properly how to use this. Where should I start? What do you call the harness behind the key ignition switch? And why can't I find a place that has the whole neg. Cable from the starter to the battery? Auto stores only have the posts thank you my name is josh