Much like leenmi, I went through a polygraph test once in connection with my security clearance. I was working in the defense industry at the time (I have since changed industries), and they were running everybody through counterintelligence polys.
(A counterintel poly is where they ask about your likelihood of breaking security and potential to be compromised. Contrast this with the full lifestyle poly, where they ask about any bloody thing they please, especially your sexual habits.)
The only thing I can add to leenmi’s description of the poly is that I kept having problems with the blood pressure cuff. They had to adjust it to its minimum tightness, and my arm still fell asleep. I tried to shake out the arm DURING THE TEST, and drew a sharp “Don’t do that!” from the examiner.
To address the OP about what it’s like: You sit in an armchair for a while. There’s tubes around your chest to measure your breathing, a pressure cuff on your arm to measure your blood pressure, and clips on your fingers to measure your pulse rate. You’re not supposed to move at all during the test, not even your eyes. The examiner sits out of your line of vision. They ask assorted questions (which IME they had already reviewed with me), and after each question they pause for a bit to see if the readings change.
Assuming the exam is routine and you aren’t hiding anything, the whole experience is rather quite boring, depending on your level of test anxiety.
People who practice biofeedback (which I myself find to be a bit whacky, but to each his own) could take a polygraph test and claim that they raped the pope with an alien anal probe, and come back as truthfull. Relaxation techniques have been shown to work as well.
I took one to be hired, some 16 years ago. I didn’t find it very stressful at all, though I agree that it was uncomfortable.
The process involved going through a long questionaire of specific questions, then I was shown a list of 15 (IIRC) questions that would be asked while I was hooked up. These were things like: Did you lie about your criminal history? Did you lie about any sexual crimes? Did you lie about your drug use?
Since I had been honest about everything in the questionaire, passing the poly was no problem. Remember, they may not care about some past problems, but if you fail the poly you’re probably eliminated. Tell the truth on everything, then the poly is no sweat.