Line up the two parts so that the colour you want in the centre is the inner part. Insert the larger blade of the outer part into the slid below the smaller blade of the other. Turn the heart to the other side and do the same here. When you weave the central part the strip is much wider than the slid you have to put it into. Make a soft bend and straighten afterwards. A pair of tweezers is a handy tool.
Please find a picture guide, illustrating the sequence of weaving.