Hey Scared
The surgury is usually done under general anaesthesia, with a colon prep beforehand. Did your doctor tell you to get some glycerin suppositories?
You'll likely be out cold for the placement, so don't worry about the actual surgury. You'll wake up with some gauze in your butt, and some funny mesh underwear on. Your doc might suggest some stool softeners. Depending on how the surgury goes, you might have stitches. If so, you should avoid water for a day. If not, you can buy a sitz bath at CVS, and use it to soak your new piercing.
You'll hopefully experience some drainage, which can be caught with a pantiliner or gauze. Also, sometimes people get fistula farts... Gas from your bowels passes through the fistula. It feels weird, but isn't painful. Don't be surprised
Good luck