There's nothing to it, all they do is hook it up to a machine, punch in a few settings and leave the machine to it. It takes a good half hour or so for the machine to clean out the system, pressure test it and refill it. With the Leon there was a choice of 3 different A/C pump manufacturers used and we had to select the right one on the machine (Had to find a small mirror to look on the A/C pump to find the manufacturer, as 'they don't usually check that and just go with the first option'!)
I had mine done in Kwikfit a couple of weeks back, along with a full hunter wheel alignment, for the grand old sum of £20 in my mates pocket