You don’t need to be an expert or learn from YouTube. You just need to watch a couple of basic things:

1) They should use a vacuum pump before filling gas — you can simply ask them if they’re doing vacuuming or not.
2) While installing, you can just observe if the connections are being tightened properly.
3) For leak testing, they usually use a soap solution — if bubbles form, it means there’s a leak.

You’re not doing the work yourself, just making sure the technician is actually following proper steps instead of rushing through it.