Foundation - Ch4, sl st into 1st ch to join. Or make a MR.
Rnd 1 - Ch3 (counts as 1dc), 2dc, ch2, (3dc, ch2) 3 times, join with sl st in top of first ch-3, turn. [12dc]
Rnd 2 - Sl st into the ch-2 sp, ch3 (counts as 1dc), 2dc into ch-2 sp, *(3dc, ch2, 3dc) in next ch-2 sp, rpt from * 2 more times, (3dc, ch2) in the same sp as 1st 3dc cluster, join with sl st in top of 1st ch-3, turn. [24dc]
Rnd 3 - Sl st into the ch-2 sp, ch3 (counts as 1dc), 2dc into ch-2 sp, *(3dc) in next sp, (3dc, ch2, 3dc) in ch-2 sp, repeat from * 2 more times, (3dc) in next sp, (3dc, ch2) into the same sp as the first 3dc, join with sl st in top of first ch-3, turn. [36dc]
Rnd 4 - Sl st into the ch-2 sp, ch3 (counts as 1dc), 2dc into ch-2 sp, *(3dc) in next 2-sps, (3dc, ch2, 3dc) in ch-2 sp, rpt from * 2 more times, (3dc) into next 2-sps, (3dc, ch2) into the same sp as the 1st 3dc, join the round with sl st. [48dc]
Finish - Once you have completed the granny square, cut the yarn from the ball leaving a 6-inch (15 cm) and pull it through the last stitch. Use a darning needle to weave in the loose ends.