I had one once. They use a very, very small, thin
needle. You get a shot of anisthetic, which hurts, but you'll feel thankful for.
To have one done, you must curl up in the fetal position, meaning bring your legs up to your chin and hug them tightly.
My advice for you. Don't Move! If the doctor messes up, there is a risk that the fluid in your spine will depressurize, causing brain damage. Thus the need for a consent form, and it's a safe but serious procedure.
I advise going to the bathroom beforehand, because you should lie down for 12 hours afterward, or you could get a tremendous migraine if you sit up.
Other than having a Band-aid on your back and a small scab, there are no longterm effects.