Step1: turn your Cricut to it's back and remove the 4 rubber feets on the bottom of your cricut
Step 2: unscrew the 4 screws you'll find under the rubber feets
Step 3: put the stacker in place and pay attention to the "front" mark at the stacker
Step 4: screw the stacker tight with the supplied screws (doesn't work with the original screws)
Done!
It's the same procedure for the top and bottom Cricut. But when you install 2 Cricut maschines, start with the top one. I also would suggest to keep the original accessories.