I'll try and answer each of your questions in order!
* It depends re keeping pants on. If you are having it all over, before going on hols, then you might want your your bum doing to IYKWIM. If you're just having your legs done, then there's no reason to take them off. You're covered with a towel the whole time anyway, they just move it from side to side as they need to.
* There is a special lotion that goes on your face, lighter than on your body so that you don't look like a clementine! They will ask you though, so if you don't want it, you don;t have to, but well worth for the exfoliating facial if you ask me!
* They will advise you re shade to best match your skin tone.
* As long as it is properly applied, and if you are having it professionally done then there is no reason why it won't be, then it won;t be streaky or patchy. Just looks a bit yukky until you can havea shower.
* Make sure you use plenty of body lotion, but you can't stop it from fading, although you could have another one a little while after to intensify the tan.
* Depends, but IME normally about 5-10 days.
* Make sure you wear some old baggy clothes (without a tight waistband) to go home in and depending on what time of day you have it done, if you can't have a shower before you got o bed sleep on a towel.
Or you could just forget paying out all the money, and do it yourself at home with Johnsons Holiday Skin, if you apply the lotion every other day, it lasts and lasts and looks really natural - I LOVE IT!