What's the best way to measure for tire size.

I measured what I had so I had a baseline. Then I measured and added the space I had left. Make sure you measure both sides. They are usually different especially the rear. When I got my wheels, I was lucky enough to find a used tire the size I was planning to buy. I put it on the rim, tried it for fit on both sides. Then I ordered the new tires. If you pull the trigger and order new ones, you might not be able to return them if they don't fit.