You crochet your number of stitches for the length you want.
I will tell you in DC.
Row 1- chain 2, dc in each stitch across, dc in last stitch.Ch 2 turn,
Row 2- fpdc, bpdc alternately across dc in last stitch. Ch 2 turn
Row 3- bpdc, fpdc, alternately to last stitch dc. Ch 2 turn,
Repeat row 2 and 3 until you have length you desire for the ribbing.
Hope this helps.
Hokeyman