Here are the design principles I used:
1. Corners need 3 sc or 5 dc to be square
2. It's easier to feel a chain 1 than to feel the middle stitch
3. Dc are twice as tall as they are wide
With Color A, chain a number of chains such that the length equals the difference you want in the length and width of the finished product. For instance, if you want to make a twin-sized blanket, typically 75"x90", then the difference is 15". Your chain should measure 15". This is only an approximation because work does tend to stretch the starting chain, but it's a starting point.
Round 1: dc in 4th ch from hook and in each ch across with color A. Attach Color B to back side of chain (at the beginning of the chain, where the little bit of string is hanging off, where you've just crocheted back to with Color A). Ch 3 and dc across.
To make a corner: (2 dc, ch 1, 2 dc) in last ch
To make sides: dc in post of first dc of opposite color. Make corner in top of first dc of opposite color. These sides should be 5 dc in length (2 for each corner and 1 for the height of the initial dc)
Round 2: dc around with (2 dc, ch 1, 2 dc) in the ch 1 of corners
Repeat Round 2 until work reaches desired size.