Not too bad! As someone who has donated bone marrow, I can tell you that the procedure is quite comfortable, and the next day felt like mild low back pain which I was able to control with tylenol (acetaminophen). Good luck!
There is a lot. Of variation from patient to patient. It is important to get the surface of the bone (periosteum) well numbed. There is a sensation from "tug" to "significant brief discomfort" when the liquid is aspirated from the marrow space, but it only lasts seconds.