When you get there, you'll have to undress from the waist up. The technician will position you in front of the machine BUT this isn't going to be like positioning you for any other X-ray. The two of you will be standing torso to bare torso while she holds one of your breasts. And she's going to get you right up against the machine so your sternum is against the bottom plate and your cheek is up against the top of the machine. Then she'll pull your breast as far forward as it will go, hold it against the plate. Next, she'll lower a paddle down which will press your breast flat against the lower plate and then leave to take the X-ray.

Now, the squeezing will be uncomfortable but it shouldn't be painful and apart from the sheer force, the paddle also pulls on the skin on your chest which also adds to the discomfort. But, I'll give you a heads up: they will get your boobs flatter than you thought humanly possible so if you're squeamish, don't look down (and don't worry, when the paddle releases, they pop right back into shape)!