The first one
A first haircut goes well or badly on the twenty minutes before it, not during it. Four things decide it and none of them are about hair.
Come when it is quiet. Tuesday or Wednesday morning. An empty shop with one barber and no queue is a completely different room from a Saturday at eleven with six men waiting, and a two-year-old can tell the difference immediately.
Do not warn him for a week. A child told on Monday that Saturday is haircut day has five days to build it into something. Tell him that morning, casually, and make it an errand rather than an event.
Sit in the chair first, yourself. This is the father-and-son cut and it is why the price is what it is. He watches you have it done, in the same chair, by the same man, and it stops being a strange machine near his head.
Let him keep his own hands busy. The toy chest is by the window and everything in it is allowed in the chair.
What we will not do
Hold a crying child down to finish a haircut. If it is not going to happen today, it is not going to happen today, and we would rather he comes back in a fortnight and enjoys it than we get a tidy result and lose him for ten years.
There is no charge for an attempt that does not finish. That has happened perhaps twice a year for thirty years and every one of them came back.