Granny stitch consists of groups of 3 treble crochets (US double crochets) worked into spaces between groups.
Each row is different from the previous, and it can be tricky to get started. This tutorial will show you how to make a swatch 8 groups wide and 12 rows high. To make it wider, simply add 3ch to your foundation chain, for every granny stitch group you want to add.
R1: 3tr in 5th ch from hook, (miss 2ch, 3tr in next ch) 7 times, miss 1 ch, 1tr in last ch, turn.
R2: 3ch (counts as 1st tr), 1tr in sp between tr and 3tr group, 3tr in each of next 7 sps, 2tr in sp between last 3tr and 3ch, turn.
R3: 3ch (counts as 1st tr), 3tr in sp between 2tr and 3tr group, 3tr in each of next 7 sps, 1tr in sp between last 2 tr, turn.
Alternate between R2 and R3 until you have 12 rows.