A myelogram is done to see if there is compression of the spinal cord or nerve roots and if so, how badly.
It's not a fun test, but it's not nearly as bad as it is made to sound on the internet.
They will have you come to the hospital a few hours before the test is scheduled to get you set up in a room and then down you will go to radiology. They have you lay on your side or stomach, whichever is easier for the radiologist, and then they give you a small injection of lidocaine or something similar, wait a few seconds and then introduce the needle into the spinal cord. It is really important that you don't move during this part.
They withdraw a small amount of fluid then inject the dye. You can actually watch it on the fluroscope monitor by your head.
After that is done, they take a few xrays right there, and transfer you on a gurney and take you over to the CT scanner for a few more pictures.
All in all, I think it took about an hour for mine, each time.
After that, it is back upstairs for a period of time before you go home. The biggest thing you will need to remember is to follow the directions of the nursing staff once you go home.