I glue, either with stick glue or Elmers School. Use just a tiny bit at each place. One of the neat things about using glue is you can make detailed things like hummingbird or flowers, or, in the case of one of my grandchildren, totem pole images of Raven or Eagle, etc. and get them exactly where they should be, and they'll stay until you sew. These all come completely out when washed.
Hint for grannies: help a child find a picture s/he finds exciting (not toooo detailed); use tissue paper to make templates (we don't have a printer); go to your stash and choose bright plain colors and let them choose (provide guidance when necessary because you want the child to really like the outcome; teach how to cut the patterns, (for really young or unsure children it can be better to not have seam allowance, for older the can understand why); help them paste the cut pieces on to the chosen backing; and bingo, your little one has a wonderful picture to give Mommy or Daddy and they are so proud! AND you have used a bit of your scraps. I've never done it, but they could design quilts this way also and make mini quilts for their paperdolls or rungs for their doll houses, or.....