Using an Iron: When using an iron, use the cotton setting.Press on a hard surface using a bit of pressure (not a folding table or ironing board.) Press and hold, pick up iron and move just a bit. Be sure to move it just enough so that the preceding steam holes are covered. If you see parts that are still opaque, repress. It is not hot enough. There is such a thing as too hot as glue will start to dissolve into fabric.
Do not place iron or heat press directly onto a design. If the foil cover sticks to the shirt, you can pick it off with tweezers. The heating element got too hot.