Hi, PN
My ds had this done when he was 16 months, which seems to be the standard age at which it is done.
It was done as a day-case surgery. We went in quite early in the morning, with ds not allowed to eat anything since the previous evening. Tbh, that was the bit I was most worried about, but actually it wasn't too bad, as he just became lethargic rather than screeching the place down with hunger. After being checked in they put a patch of anaesthetic gel on his hand, covered with a piece of sticky clingfilmy stuff.
We then had to wait an hour or two before it was his turn, which did get a bit tedious, as he wasn't in the best of tempers. I just walked up and down endlessly and sang little songs to him.
When it was his turn I was allowed into the theatre ante-room for the anaesthetic, which seems to be standard from what I've heard. They put a needle in his hand which he didn't notice at all because of the anaesthetic gel I held him so the hand was dangling down behind me, out of his sight. I then had to put him down on a trolley while they administered the full general anaesthetic that was the worst bit, as they look so little and vulnerable. He also went a bit bluish, which is apparently common, and not dangerous. Then I was ushered out, and had to wait till it was all over.
Parents weren't supposed to go in till the babies were awake, but my ds was such a sleepy bunny that they let me in anyway. I had a bottle of milk ready, and when he started to wake up I gave him the bottle, which he drained and then went back to sleep it off. And that was it, really. We went home a few hours later.
Afterward you have to keep the nappy very loose, and give painkillers prophylactically -- ie. before the pain has a chance to build up. IIRC we were alternating Calpol and baby Nurofen, pretty much as you would for a nasty cold. We did that for a couple of days, and he never seemed to be in any real discomfort. The scrotum looked a bit bruised for a few days. He had a tiny incision in his abdomen, and another tiny incision in the scrotum. Both are now invisible (he's nearly 6 now). The testicle has stayed down. He had follow-up checkups for a year or two. He knows he had the operation, and knows that he must check his testicles for lumps every so often. We've had no other problems either with the operation or afterwards.
hth