Size J hook (6mm)
In the 6th ch from hook, 3dc, skip 2 chains, 3dc in the same chain to last chain, 1dc in last chain, ch3 (last ch3 counts as 1st dc on the next cluster).
In the space on the end created by the dc; 2dc, ch3, 3dc, ch3, 3dc.
3dc between each 3dc cluster to beginning loop made by foundation chain, 3dc, ch3, 3dc, ch3, 3dc, sl to join.
Keep crocheting around this rectangle using the granny square stitch till you reach the desired size for your afghan.
The starting rectangle above is sized to crochet baby and crib sized afghans. I have not yet tried it for a full size afghan, to get your lengths and widths to work out. It wouldn't be hard to modify it though.
Block if so desired.