Simply start like crocheting a granny square. Join every round with a couple of slip stitches so you don't have to break the yarn. In round one crochet 2 ch for each corner, in round two only crochet 1 ch for the corner. Also instead of the 1 ch between the 3 dc hook one dc on two sides in the second row.
*** Find this pattern @ Ravelry: Uta's Granny Hearts ***