Having seen the ISPY pillows on etsy for quite some time, I decided to try my hand at it. Making them is really easy!
Since many have already posted directions to make the fleece bags, I'll just refer you to According to Kelly... if you would like to know how to make them with fleece.
If you want to use flannel, there are two ways. The first is a patchwork style and directions can be found at The Enrichment Cottage. The way I did it requires that you make small diagonal cuts in the corners of the flannel where you are putting the "window" and folding the edges under. Run a seam around the window, then sew the vinyl in place.
If you have trouble knowing what to put in the bags, consider doll house sized items from your local craft store. I also included some beads, small erasers, paper clips, scrabble tiles, water balloons (empty and tied closed), jacks, dice, buttons and googly eyes. I also purchased some trinkets from HighPie on etsy.