A good fitter will ask you a lot of questions about what kind of riding you do, how much you ride, how long your rides are, any physical problems you've had, etc. The fitter will also take a bunch of measurements while you're off the bike, including some movement exercises to determine your flexibility. Then they put your bike on a trainer and watch how you pedal, make adjustments and repeat as necessary.

I've had a professional fitting done on two different bikes. Both times I went in kind of hoping for a cool-looking bars low/saddle high kind of sporty riding position, but I answered their questions honestly and came out with handlebars about level with the saddle and a relatively upright riding position (by drop bar road bike standards).

The first time I had been riding the bike with the bars low and went in hoping to relieve some numbness issues I'd been having. The first couple of rides after the fitting felt like driving a bus it was so different, but it fixed the physical problems I was having and once I got used to it, it was super comfortable. I still use measurements I took from the fitting as a baseline for all bikes I set up for myself.

For my second fitting I had just bought a snazzy new carbon race bike and was hoping to get set up with a comfortable yet aggressive position. I went to a different fitter who used a totally different system than the first. I went in with the bike barely assembled, not even attempting to match the fit of my other bikes. I came out with a riser stem and a fit that almost exactly matched the other one.

Based on these two experiences, I have a good bit of faith in professional fittings. I believe that these fitters at least were dedicated to setting me up with a fit that would work for me personally, prioritizing that even above my romantic fantasies about pretending to be a racer. As I recall both offered an extended comfort guarantee. I just looked at the web site for one of the shops and their guarantee is for one year.

It's a decent description of the process for anyone considering a bike fitting:

Bike Fitting Services | Bike Gallery