The card base is a 6x6 square, and I followed this tutorial for cutting and remodelling it. The two front flaps are held in a bend by tabs that fit in slits. The flowers were then stamped both onto the card base and onto book paper. After quite a bit of fussy cutting, the pieces were in place and I added a touch of white gel pen. Here's how it folds out.
I can imagine adding a sentiment like "congratulations" or "thanks" on the innermost layer, perhaps stamping or heat embossing it with white on a piece of kraft card. For the time being I'm leaving this as it is. I like to do a bit of mail art while I have the stuff out. Since it's not certain that glued pieces would stay stuck on an envelope, I just used a gel pen on a stamped flower. When the tabs are pulled out of the slits, the card fits in the envelope (just have to hope the recipient realizes how to assemble the card!)