I use a cirlce template. It cost me about 2 dollars, maybe 3. If I get a little paint on it I really dont care
I put a little bit of tape around the circle I am using incase there is any overspray, but it is minimal.
If the opening for the tire is a little to big, do not worry... you will not notice it. I painted these panther tires all black while they were on the sprue. Then I cleaned them up of any flash and touched up the black paint. I found the best fitting circle in my template, and sprayed the rim in a circular motion with my airbrush.
The overspray on the tire is not visible (the circle was a teeney bit too large). Give it a shot and you should have great results on the first try