This is not an easy task and beginners of impressioning should save their time and money on key blanks and put effort into impressioning file cabinets or desk drawers first. For those of the site that are masters if not the yoda's of impressioning here is a little quick setup you can do to impression a medeco. I'll be the first to say it's really hard.
Take a key blank Ilco xxxxxxxxxxx shave a tiny bit off the bottom and a tiny bit off the top (run the key through a key cutter and hold it steady to slice off a thin layer off each end, take off the thickness of half of a penny. This blank will now slide perfectly top to bottom into a Medeco Cylinder and the key is not restricted. You now made a perfectly good medeco copy cat blank for about 18 to 24 cents wholesale locksmith price. Now impressioning has to be done at Angles and this is the toughest part about it, i can make a guide with pictures but as i said it's not really easy, having a cylinder without the retaining tail piece or cam on the back to check your progress taking the cylinder out (using the real key of course) and looking at the levels of the pins and the rotation of them. The first few times you impression you will want an existing key side by side to moniter your shapes and make sure your marks are correct and how to identify the slants and angles of the marks, checking the existing key and looking at your own. with enough practice and being able to identify the ghost marks on the key and identifying the real marks eventually you will impression Medeco Keys. Good luck it's no easy task.
I've had one guy in my shop do it on the spot with a sealed cylinder, but he's the yoda of the locksmith gods.
edited by zeke79 to remove blank info.