Martha has the instructions on her site for crayon hearts but I'll take you through the process.
First, we made the crayon shavings with a pencil sharpener. A bit of a messy job.
Then I melted the crayons between 2 sheets of wax paper. Remember to put a towel underneath the wax paper or you will have crayon melted to your ironing board (ask me how I know this) and brown paper between the iron and the top sheet. If I was to do it again, I'd spread out the crayon more- a little goes a long way.
Then we made up heart shapes with regular paper for templates and used those to cut out the hearts from the wax paper. Next I used a needle and embroidery floss to string the hearts. To show them in the window, I used a short piece of wire hiding strip- it has an adhesive strip to hold it in place.
And viola! An easy Valentine's Day decoration!