I know it would be much easier to show this in a video but I'm not there yet so I'll do my best to demonstrate this through words. Even after I saw it done in person I still had to cut and recut portions of my card to make it work so you may want to experiment with scraps first.
Using Stampin' Up's window punch, create two slots on the front of the card. You will need to make sure that you can punch from both sides and you may need to lengthen the slots depending on your design.
Next, place the cardstock for the popup that will appear at the back of the card and align it with the very top of the inside card base. Unfortunately I cut my first popup too narrow so I had to add cardstock scraps to the bottom so it would show through the slots.
Then close the card base on top of the rear popup and place two stampin dimensionals at the bottom of each card slot making sure they stick to your rear popup. The width of the dimensional matches the width of the window punch perfectly so if you don't have these you will need to experiment.
Now you're going to adhere your next popup to the stampin dimensionals. You'll need to be careful to make sure the height of this popup doesn't block the popup in the rear. If you can't tell I had to make some modifications to my second popup as well. Keep in mind that you'll need to cover this second popup with a final piece of cardstock while still leaving room for it to move up and down.
In order for the popups to slide you will need to raise the final piece of cardstock above the popup on the front. Once again adhere stampin dimensionals. My original plan was to center this on the front of the card base but I had miscalculated the length of the slots and had to move it down to the very bottom.
The last step is to glue the sides of the cardstock together making sure you don't obstruct the rear popup on the inside of the card base.
To finish the card you'll need to attach ribbon or some other embellishment to the top of the rear popup so you can pull it up from inside the card base. Below is a picture of the tools I used.