There are many patterns for this blanket out there, but this is the one I'm using - it doesn't 'belong' to me it's just my interpretation. I have never written down a pattern before or photographed it step by step so I hope you can understand it. I am using the US terminology for the stitch names.
With colour A, make 4 chain stitches and join into a circle.
Make 5 chains (this counts as the first treble and chain)
Make 1 treble (1 tr, in the circle and then 1 chain. )
Repeat until there is 12 trebles in the circle and join yarn with a slip stitch to the first treble to make a circle.
Join colour B and make 3 chain stitches ( 3ch). This will count as one double crochet (dc). Make another dc into the space of the circle, then 1 ch.
Make 2 dc in the next space and then a chain. Repeat all the way around the circle with 2 dc with a chain inbetween.
Slip stitch onto the first dc to form a circle. Now slip stitch through the next stitch (this positions the yarn closer to the next space where you will change the yarn to colour C)
Chain 3 (this counts as 1 dc) 2dc into space, 1 ch, then 3dc followed by a chain into next 11 spaces.
Slip st into the first dc to form a ring. Now slip st through the next 2 stitches to make the yarn closer to the next space.
Change to yarn D (white) Chain 3 (this counts as the first dc) then 2dc into the space, one chain, then 3 dc into same space (this is one of the corners).
*1 chain, then 2 dc into next space, 1 ch and 2 more dc into the same space*, repeat from * into next space, 1 ch then at the next space (the next corner) make 3dc, 1 ch, 3dc.
I know this 'square' has different colours but it gives you an overview of the final round. The 4 corner spaces have 3dc, 1ch, 3dc and 8 side spaces have 2dc, 1ch, 2dc. All are joined by 1 chain.
When you have made one square you start another one; when you get to colour D (white) you can join the 2 by slip stitching them together when you reach the chain stitch in each space.