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Here's the skinny on making an embroidered heart with you and your fiance's initials. While I made the dress as well, this is something you could do to a store bought dress, though I imagine it would void any kind of return policy. For this you're going to need embroidery floss in whatever color suits your fancy, a large eyed needle, a piece of paper and some basic art skills or a computer.
Step 1Making the design
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Since you're embroidering the dress directly, you want to make sure of two things: 1, that you don't screw up, and 2, that you don't leave marks all over the dress. I did a test run on a separate piece of fabric before starting to see what this would look like, just freehanding. Seeing how lopsided my heart was, I knew I wanted to actually plan better for the real deal.
I had no plans of drawing on my actual bodice, regardless of how well the marking pen says it will come off. Instead I drew my design on a piece of paper that I then pinned to the front of the bodice. For anyone who has gotten a tattoo, that's what I felt like I was doing as I started sewing.