(affiliate links provided for your convenience):
To mix the colors, I used a "technique" that I use since I was a kid: the snake! Roll each color out about 4 inches long. Twist each color together. Fold and then twist again until the colors are blended the way you want.
Form a ball with the clay. Use a wooden rolling pin
to flatten the clay then use the cookie cutter to make your hearts.
In case you want to make a necklace with them,
Bake them for 15 minutes and you'll have a
beautiful collection of hearts to make a necklace (or a keychain or a charm or whatever you want), hand them out to friends or leave them around as a piece of art + kindness abandonment. You can also add a positive word on each one of them and make them even more special.