Don't use your best paper for the first try. I can almost guarantee you will end up throwing it in the trash. So here goes.
Start with one 12 X 12 piece of cardstock-
Cut 2 pieces that are 12 X 5.5"
At one end make a mark at 3" and then cut from that mark to the corner. It should look like this: Cut both pieces exactly the same.
So far both pieces should look the same but that changes now.
On the first piece cut from the bottom up to the center line at 2" and 10". Next cut from the top down to the center line at 6".
On the second piece cut from the top down to the center line at 2" and 10". Next cut from the bottom up the center line at 6". See below:
If you've made it this far, it's all down hill from here.
Fold on the score lines.
Next slide the cuts together until the bottom is level. It may feel a little awkward but all the cuts will slide together to make two closed sections.
It should fold flat for decorationg and mailing - have fun and good luck!