This seems like a decent tutorial https://youtu.be/ORv2hInGB0Y

You can do this without the clippers and just use scissors. You can get a cheap pair of hair shears at a big box, drug, or beauty supply store. It will make it easier than using regular scissors.

If you’re not going super short you don’t have to worry about being very exact since curly hair hides small imperfections in the cut.

I would recommend checking for symmetry often- making sure the same pieces on the left and right sides of his head are the same length, and stepping back to see how it’s going- don’t rush it. Also if you’re not sure if you should stop or cut more, stop and let him wear it for a day or two, you can always trim more later.