i had one last year, they take you in as a day case, no need to stay over night. after checking you are actually the right patient and you sign the consent stuff, they get youto take off all metal items, including your bra, then once in the theatre, they put in a canula to give you sedation and give you some spray in your mouth, a bit like breath freshener to numb it, after a few minutes once it works they put in a thing to hold your mouth open then ask you to try and swallow the tube (i really struggled with this and they had to give me extra sedation before my throat relaxed enough...it didn't hurt though) i think i dosed off while they were looking cos i don't remember much and woke up as they were finishing, after wards i was totally ok if a little sore throated for a day or so.
the worst thing for me was they had to hold the canula in my arm using a sticky patch which i am allergic to so i had a big red patch of raw skin when they removed it! but there again i am used to that as it always happens with even hypoallergenic plasters!

hope it goes ok x
take care x