Howdy June! Try not to worry too much about this. Although no procedure is completly without risks, this one is done routinely and frequently in most hospitals, and the docs are all old pros at it.
You will be sedated. The doctor will thread a cath line through a very small incision in your groin into the large vein there. They will then follow the blood vessels to your heart.
Once there, various contrast dyes and other tests can be conducted while the doctor observes how blood flows in and around your heart. My husband has had two, neither one of which took longer than 30 minutes.
I had a similiar procedure, except they looked at the blood flow in my brain instead of the heart. They are correct that the worst part is the laying still for three hours afterwords to make sure you don't bleed from the incision site.
Myself, I took the opportunity to take a nap, one of my more favorite pastimes these days, and one I don't get to participate in nearly enough! Your doctor should have results for you before you even leave the hospital.x
Compared to having lupus, this one is easy!