Just put the smaller parts in different colour on top and weave as usual.
Here are some more detailed instructions.
1) Cut out the three pieces in the two colours you prefer.
2) Crease all pieces along the short centerline.
3) Wrap the narrow pieces around the solid of the other colour to form the heart’s two parts.
4) Place one part behind the other.
5) Weave the wider strips as usual. The narrow strips just go along.
And a file for cutting: heart 007 cut