These are the invitations I made for Daphne's sleepover. They turned out really cute and were sooooo easy. I bought blank cards, with envelopes.
1 - I trimmed about a 1/2 an inch off the open side of the card to make it closr in shape to a sleeping bag.
2- I decided how much of one corner I wanted to fold over and using a ruler and the dull edge of a butter knife I scored the fold line.
3 - Then I took the card to my sewing machine and starting below the score line I sewed about an 1/8 of an inch from the edge down the long side and across the bottom. You could use double sided tape!
4 - Next using a basic word program I printed the invite info on colored paper. The only things to figure out are how wide a piece of paper will fit into the sleeping bag. If you use double sided tape it will be thinner. You also need to figure out how low the words need to be so they don't show above the bag.
5 - And finally I found a free clip art image of a pillow, but you could just draw a simple pillow shape, and printed it onto another colored paper. Cut it out and using double sided tape I attached it to the top of the insert. Just make sure that it all fits into your envelope.