Done in jumper weight, 3mm needles. No seams. 24 stitches, 48 rows garter stitch.
With the right side facing you, join the new colour to the edge of the previous square by picking up the thread through each of the garter stitch "bumps". The new square will be knitted at a 90° angle to the previous one. On all even rows, join the new square to the previous one at the end or beginning of each row by knitting together the live stitch of the previous square with the edge stitch of the new square. When all live stitches have been joined to the new row your new square if done. Leave the last row stitches live.
To reduce the number of loose ends, weave in the new colour's loose end on row 3.