I assume this is a routine, baseline mamo for you. Some people will tell you how painful it is, but I have had a half dozen or more and never experienced anything more than mild squeezing during the actual test.
At St. Mary's Hospital, they are very efficient in the process and you can actually be in and out of the hospital in less than 30 minutes. You preregister on the phone before the day of your appointment. They take you into another section, where you have a private dressing room, to remove everything from the waist up and put on their gown. The techs who perform the mamo are all very friendly and professional. They warn you if they think you might experience some discomfort, and sincerely seem to care about you.
No, they don't flatten you entirely--thankfully! They just arrange your breasts (one at a time) between two flat surfaces, to maximize the area to be photographed. The tech steps out during the actual 'photo,' to reduce exposure to herself.
I usually get a report from the hospital within two weeks of the mamo, telling me everything checked out as normal.
Glad to hear you are getting it done. And don't forget to follow up routinely at intervals suggested by your doctor.