I've used a standard double knit and a 4mm crochet hook.
For your next round start with the ch3 and treble 2, ch1 and then treble 3 in the next ch1 space. After your treble cluster ch1.
Your corners will be exactly the same as you worked in the second round.
The chart shows two rounds only. The dot usually indicates a slip stitch but I've added it to show you where the joining treble goes.