Make sure your car runs reasonably well unless you are doing it to get something fixed or diagnosed.
Check oil level, air filter, plugs, coolant, tyre pressures, fuel and anything else you can think of.

Car gets strapped on, they should check compression and other things just to make sure the car is sound and won't blow up, they will then connect all sorts of engine monitoring probes, maybe plumb in a boost gauge (a precise one !).
Then, the car is run and fettled until she runs her best.

How long it takes depends on what you are having done, but I guess 1 to 1.5 hours for a tune should be sufficient.

Had my cossie done for £90 last year + parts and fitting !!!

Good luck