As I stated in this post, I never tie dye, so it's quite possible that there is a better tutorial out there somewhere, but this is what worked for me :)
The pictures pretty much tell you everything you need to know, but I did learn a few things along the way, so I will include a few notes.
1. Draw your shape on your fabric with a washable marker. And then don't forget to wash out the marker before putting the rubber band on in step four. Trust me, I learned the hard way that once you tie dye washable marker in boiling water it is no longer quite as washable :)
2. Stitch around the shape with a simple running stitch. I found that my stitches were a little to close together and that I could have saved some time by spacing them out a bit more. It still worked, but it could have been easier. Make sure that both ends of the thread are on the back of the fabric. Also, use a thread that matches your fabric. I used a contrasting thread for the photos but in the end a little of the color permanently transferred to the fabric.
3. Pull the strings to gather the fabric along the shape. You can leave the thread in the fabric while you tie dye and it will not affect the final product.
4. Wrap the rubber band tightly around the gathers. Try to make sure that the gathering stitches are right under the rubber band or your finished shape may end up a bit wonky :)
5. Tie dye using the method of your choice. I submersed the whole thing in a single color, but you could also dye the heart a different color than the rest of the fabric and it would probably be even cuter :) Make sure to rinse your fabric thoroughly according to whatever tie dying directions you are following before removing the rubber band and gathering thread. Then you are done! Or, if you are like me, you just started since you still have to sew an entire outfit :)