I bordered the square with a round of half double crochet (UK half treble) in white and then a round of double crochet (uk treble) in red.
The back is all grey C2C crochet with the same two additional rounds as the front, but all in red.
I then joined the front and back with a round of single crochet.
I'm going to give him to my little nephew, as I'm hoping he'll like a cheery santa cushion!
For all details on pattern, yarn, and tips used to make the santa square, see this post.