Train yourself to have good timing
Consider this before you train your dog - the better you are at training mechanics, the easier it is for your dog to learn and the quicker you get to awesome results.
Most of your training will depend upon you marking your dog for making good choices (ones that move your dog toward the finished behavior you want to see), and reinforcing those choices with something your dog thinks is awesome (treats/toys/play).
It is easy to anticipate seeing what you want to see, getting excited and marking behaviors before they happen. Or you may find yourself marking any and all behaviors instead of differentially marking the ones that you really like.
This approach will also help you practice the mechanics of using a very effective kind of maker called a “clicker” (practice using the clicker with both hands). Another effective marker is your voice - practice using your voice as a marker with a word like “nice” or “yes” that you can say consistently.
Also practice your timing and placement of rewards (in this case, by putting treats in a bowl). Notice that marking the behavior is not the same as rewarding. Also notice that marking is not the same as praising. Here I am not using praise to encourage or motivate the tennis ball, but I could: “good job little dude!”
As a final caution, being only ok at using a marker can be worse than not using it at all. Because the marker is such a clear way of communicating with your dog, it only takes a few repetitions of marking the wrong thing or having bad timing to build a real problem for yourself. Here are some very common agility examples:
1) accidentally training your dog to look backwards going over a jump - comes from waiting to throw a reward until after the dog clear the bar.
2) accidentally teaching the dog to pop out of weave poles early - comes from reaching for your reward (into a pocket, or for a toy) in anticipation of the dog finishing the poles.
3) teaching your dog to be confused about contact performance - comes from accidentally reinforcing the dog for checking with you on the way to their spot.