The prep would take about an hour -- they'll want to do some checking of your vital signs, prepare the area (sanitize), presumably give you some instructions about laying flat for several hours afterwards, how to try to avoid the dreaded headache, all that.
But the test itself is simple: you lie on your side curled in the fetal position, you feel a bit of pressure, experience a "pop" sensation in your lower spine. They withdraw a vial of fluids, then tell you to lie FLAT for several hours... FLAT. No pillow. And hopefully you'll be fine. The actual procedure shouldn't take more than 10 minutes or so.
If you can get a lot of fluids in you..water will work, but caffeinated beverages are better..you should be fine.
I've had 3..for various purposes, one to diagnose MS, the others for other things..without any complications. In fact, after the first one, got up and went back to work..didn't know I was supposed to lie flat!..and other than a bit of a sore back, had NO complications. I hope yours goes as uneventfully.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....