I started with coloured squares that are 2 rounds of this “solid granny square”: http://www.ravelry.com/patterns/library/simple-solid-gran...
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chain 6, join with sl st to form a ring

round 1: ch 3, 2 dc in ring, tr in ring, * 3 dc in ring, tr in ring, repeat from * twice more, join with sl st to the top of beg ch-3

round 2: ch 3, * dc in each dc to tr, ( 2 dc, tr, 2 dc) in tr, repeat from * three times more, dc in each dc across to beg ch-3, join with sl st to top of beg ch-3

repeat round 2 until square reaches desired size
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I started with chain 4 instead of 6.
Then a round of single crochet in another colour, and then 2 rounds of double crochet in cream.
Then the squares are joined with the join-as-you-go method with 6 granny clusters per side.

(chain 6, join with sl st to form a ring

round 1: ch 3, 2 dc in ring, tr in ring, * 3 dc in ring, tr in ring, repeat from * twice more, join with sl st to the top of beg ch-3

round 2: ch 3, * dc in each dc to tr, ( 2 dc, tr, 2 dc) in tr, repeat from * three times more, dc in each dc across to beg ch-3, join with sl st to top of beg ch-3

repeat round 2 until square reaches desired size.)