Not a professional but Ive DIYed it. So the following may be oversimplified and/or not best practice.
You'll need a gauge set so you can watch both the high and low side pressures. Your unit probably does not have ports to attach the gauges but they make some adapters that bolt on and pierce the lines. These work ok but will probably be another spot for leaks down the road.
You'll need to figure out what refrigerant your unit has in it, there's usually a metal placard somewhere that tells you this. This is how you'll look up where your pressures need to be.
Finally you'll need to source said refrigerant, which you likely cannot buy without a license or bribing someone.
The actual charging procedure I would look around on youtube for. It may vary depending on your type of unit but it's usually pretty straightforward.