How do you sew those pre-cut plastic canvas shapes?

This may be a silly question, but I have seen the pre-cut plastic canvas shapes in the store in shapes like hearts and stars and circles. They look cute enough to make a few ornaments for my tree, but I don't see any instructions on them as to the correct way to sew yarn or metallic thread through them. Is there a specific way your supposed to thread the yarn through them to get the best possible look? Are there instructions to these somewhere that I missed? I do sew and crochet and knit so I am not totally without any craft background, but I wanted to make sure I brought out the best in these if I tried some. I was even thinking of adding small beads or sprinkling a little glitter on them after they're done. Thanks.