Start my cutting out a heart shape from your styrofoam tray. I used styrofoam trays that I saved and washed from purchasing meat. You can mark out your heart first to find a shape and size you're happy with. If you want all your hearts to be the same size, create a pattern or use a heart template for all of them. I kind of like that all my free hand hearts are slightly different shapes and sizes.
Once you have your heart cut out, start winding the yarn around it, holding end of the yarn to start, and securing it as you wind around it tightly with more yarn. You can also use a small piece of tape on the end of the piece of yarn to hold it in place, if you prefer.
Begin by wrapping the yarn the yarn around the foam, until the whole thing is covered. Wrap randonly or start with the top of one side of the heart, and use the yarn that you've wrapped to hold the next piece in place.
Next, wrap the other top side of the heart.
Then wrap the bottom section of one side.
Next wrap the bottom section of the other side. You may need to add a few layers to the very tip to make sure it's completely covered.
Finishing wrapping the top of the heart each side.
Lastly, wrap the yarn around randomly a few times to create a more random pattern on top.
To finish, tie the end of the yarn in to one of the other pieces tightly to keep the yarn from unraveling and cut off the end.
If you want to hang your yarn heart, you can add a thread for hanging. Just cut a piece of thread, and thread it onto a needle. Use the needle, and push it through the top fo the heart. Tie the thread together, and snip off any extra thread, and it's ready for hanging.
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Notes
If you don't have a styrofoam meat tray to upcycle, you can use any stiff material like a piece of cardboard as your base. The cute yarn hearts will naturally be thicker in the center of the heart, making it kind of 3D.I prefer to use a thread to hang it because it isn't noticeable and kind of disappears, rather than a long piece of twine or yarn.