This Instructables provides a few tips learned on a recent roof repair. They worked for me, may or may not work for you. But I'm pretty sure there will be some things here you will not see elsewhere.
I started with the venerable blue tarp on my garage roof. Where I live in Washington State, it started raining last week. May stop next July.
Step 1: Pinpoint the leak source
Next, I taped a stack of rare-earth (neodymium) magnets on the underside of the roof at the high point of the leak.
Then I went back on top the roof with a cheap compass. Knowing approximately the area of the leak, the compass quickly zoomed in on the magnets under the roof. To confirm, I placed another magnet on the roof to mark the spot.