BN, "One session" is one appointment where you get all the specified bits done in one go. (I have underarms and brazillian done in one appointment and it's a fantastic treatment). I've strung out 5 appointments over 12 months. That got rid of 85% of the hair. Last appointment was November and it really does need re-doing now, but last year, when I was having the appointments more frequently, I could go 6 to 8 weeks between minor shave touchups.
they should have told you to shave the areas carefully 24hr to 12 hrs before your appointment. Apply a soothing creme after you've shaved so the skin is as calm as possible. Don't shave an hour beforehand, it will hurt more at the time. (Ask me how I know, eeep). I take two paracetamol an hour before, but I'm a complete wimp.
Bear in mind IPL works by heating up the hair follicle and pretty much sizzling the bejasus out of it. So you want the existing hair (which is heated by the laser light) to be shaved as close to the skin as possible. You don't want hair lying on the surface of the skin, as the laser light will heat it up and it'll hurt like hell. Normally if you shave the day beforehand, they will quickly go over the area with a little razor for you to pickup any stubble, it's part of the routine.
It does hurt a bit but gosh it's worth it. I find that my underarms sweat less at the gym for some time afterwards as well which is a bonus. The place I go to puts a lot of cooling gel on the area afterwards, so I wear cheap cotton M&S knickers and yoga pants and go straight home afterwards and get changed, because you always end up with a bit of cooling gel on your clothes.
For the next few days, avoid very hot showers. they should tell you about that at every apppointment and give you a care sheet at your first appointment.
FWIW, I don't like to commit to more than one appointment ahead of time and I certainly wouldn't pay for appointments in advance (what if the place goes bust or you aren't happy with them?) but I'm a bit neurotic about things like that.
Enjoy the smooth effect, I cannot understand why I didn't get it done ten years ago, it's so much nicer than batlling with a razor all the time.