There are many variations of the shell stitch, but basically all shell stitches consist of a set of stitches, usually double crochet, all made in one stitch or chain of previous row.   

Open Shell Stitch 

Shell (2dc, 2ch, 2dc) 

Make a foundation chain of a multiple of 4 stitches 

Row 1:            2dc in fourth ch from hook, (2ch, 2dc) in same st, *skip 3 ch, shell of (2dc, 2ch, 2dc) in next ch, repeat from * across 

Row 2:            3ch, turn, * shell in ch-2 sp of shell in previous row, repeat from * across making a shell in each shell. 

Repeat row 2 for pattern. 

The Open Shell Stitch may vary and could consist of the following stitch combinations: 

·         2dc, 2ch, 2dc 

·         2dc, 1ch, 2dc 

·         3dc, 2ch, 3dc 

·         1dc, 1ch, 1dc, 1ch, 1dc, 1ch, 1dc (instead of skipping 3ch in 1^st row, you skip 4ch. And a chain in a multiple of 5 sts + 4).

Solid Shell Stitch 

Make a foundation chain of a multiple of 3 stitches, plus 1 

Row 1:            sc in 2^nd ch from hook,* skip 2 ch, 5dc in the next st, skip 2ch, 1sc, repeat from * across. Row needs to end with sc 

Row 2:             3ch, turn, 4dc in 1^st sc of previous row, *skip 2st, 1sc (in center of 5dc group), skip 2 st, 5dc in next sc, repeat from * across.  

Note : 3dc at the beginning of each row counts as 1dc. 

The same as the open shell stitch, the solid shell stitch may also have different stitch variations. You can crochet a combination of hdc and dc or dc and tr. You can also decrease or increase the number of stitches skipped.