My other half had this done a while back due to heart problems, as far as I can remember, they insert a needle into the groin area that is attached to a long wire, and it goes into the artery, it then releases a dye which travels around your blood stream, this comes up on a screen and shows any blocked arteries.
You then have to stay in hospital for the day, so we went in early morning and came out late evening, I think its so the artery has healed and there are no signs of damage, bleeding etc.
Im not sure if this is what you will have, but thats what happened when we went up to the hospital.
It was all pretty painless and straight forward, the only pain is the waiting around all day at the hospital.
Edit: Forgot to add, diabetes wasnt involved in this so cant offer advice from that point of view im sorry, but good luck.