This square was made during an hours train journey to Oxford:
Create a foundation chain, multiple of 6 plus 4 for the first treble crochet.
Miss the first 6 chains, work 5 trebles in the next stitch, miss the next 2 stitches, work 1 treble in the next stitch, repeat to the end.
Chain 3, miss 2 stitches, work 5 trebles in the middle stitch of the 5 trebles from the previous row, treble in the single treble from the previous row, repeat to the end of the row.
Repeat the previous row until it is the size you require.
Finish with a round of double crochets to tidy up the edges.