Ch 21, turn, dc in 3rd ch and every ch to end (19 dc), ch2, turn *dc in top of last dc of prev row, ch1, sk st, dc in next st, sk 2 sts, 5dc in next st (shell made) sk 2 sts, dc in next st, ch 1, sk st, dc in next st, sk 2 sts, shell in next st, sk 2 st, dc in next st, ch1, sk st, dc in next st, ch2, turn.
Rpt from * until you have a few yards of yarn left to make a row of 19 dcs across.