Cut a stencil of a heart as pictured. The connecting square at the top of the heart is essential for the handle.
Position the stencil so the bottom of the heart is about 1/2 inch below the circumference line you have traced on the watermelon. The V at the bottom of the heart will become your starting point for the sword-tooth cut-outs later.
Tape on the stencil and use the non-toxic marker to trace the outline.