What is a spinal tap / lumbar puncture like?

Not bad. A small needle prick in the small of the back then some pressure and that is about it.
Usually well tolerat. This is usually a well tolerated procedure and should not cause you excessive or undue fear or anxiety. I am sure you can contact your anesthesiologist or the nurse practitioner to discuss your concerns prior to the procedure. Typicall some medications may be prescribed prior to the tap or delivered through an IV to ease your stress.