Best to phone them up for a chat but for 80% of people you'd just need to give your bike details, your riding weight, and state that you need an average rider tune.
What they'll do is change the shim stack in the damper, and maybe upgrade the piston, so that the high/low compression/rebound range is right for you. With the compression working properly, you also won't have to under/over inflate the air chamber which has negative effects if you're needing to do that.
How much of a benefit you'll get will depend on how bad the stock damper is for your weight/your bike/your riding. For me it was night and day. I could always set the fork up well for one thing, but it would have at least one serious problem (harsh/wallowing/bottoming out/compacting) doing anything else. With it tuned, I don't feel like I'm making compromises any more. I sometimes tweak the LSC a bit, mostly depending on my mood, but it has no problem with lines that are a mix of small kicky jumps, big jumps, drops to flat, fast chunky tech, tight corners and long flowy berms.
I'm really happy with what JTech have done on two forks and a shock, and they were always super helpful when I spoke with them. Also, if you buy the charger 2.1 damper from them the tuning is free. There are other options though - TF Tuned and Sprung Suspension Workshop seem to have good reputations.