Use your favorite adhesive to adhere the sides together. You should now have a pocket in the front and one in the back.
Decorate as desired. I used a piece of DSP on the flap for this particular card. I stamped an image in VersaMark for the background of the front and back.
Cut a small piece of magnet (I use those giveaway credit card type magnets cut up.) Put one piece on the main part of the card and the other on the flap. This will hold the flap closed so it won't .... flap..... around.
You will need to cut a piece of cardstock to fit the inside of the pocket, leaving enough room at the top for a tab.
Decorate the back as desired.
The back can hold another message card or in this instance, something like a packet of seeds or perhaps if the envelope is smaller, a gift card, or if larger a packet of hot chocolate.
Note: You'll want to make sure the tab is on the side of the insert that is not covered by the envelope when you slide it in - not like I've done in the picture above.