Prism99

I would add fusible to the back. My choice would be Misty Fuse. To use, place Misty Fuse on top of a sheet of parchment paper, place your fusible shapes on top (right side facing up), place another sheet of parchment paper on top, then iron over the parchment paper. Let cool just a little (still want the fusible slightly warm), then carefully peel off the parchment paper from the top, turn over and peel the parchment paper from the bottom. Use scissors to trim around the shapes. Lightly fuse the shapes to your background fabric before sewing.

You could also do this with a one-sided or paper-backed fusible, but I would be sure to use something as lightweight as possible so the applique shape does not become too stiff.

Edit: You could also simply place tiny dots of Elmer's white washable school glue on the backs of the appliques, spread around a little with your finger, then iron in place. The glue will wash out later and not show at all. However, if this is a wall hanging that won't be washed, this method might not work if the glue shows through.

Edit 2: What kind of spray did you use? 505 is a good basting spray that will wash out later. You need only a light spray (spray inside a box to avoid sticky stuff everywhere) and that should not show through the fabric. Apply the applique to the background fabric while the 505 is still sticky, and do not sew until it is completely dry (you can iron it dry to speed things up).