Question

how would you make a graph testing a ping pong ball and a tennis ball going down an incline and timing it

Answer 1

First you need to record some data. Decide what variable you want to record.
It could be the times to reach each of various stations down the incline.

If you are planning on using your eyes and a stopwatch, you'd better have a very long incline.

You could use a video of the experiment with a digital clock in the background

Some physics labs have special equipment for recording this kind of data.