Having the manual for whatever you work on is important. It's a great help for identification and part numbers. Next, a camera to photograph whatever you're taking apart. It takes all the guesswork out of how parts go back together. You can't take too many pics if your new to pinball repair.
As far as troubleshooting goes, most repairs are common sense. Your right flipper broken ? Lift the play field and look at the coil. Half the time you'll see a broken wire connection. Re-solder. Problem solved. Same flipper won't return ? You'll probably find a hanging spring. Most repairs are that simple to figure out. Good luck !