The squares are 20cm square, FolknNorah - and to be honest, they do curl up a bit at the corners - I squashed them down flat before I took the photo! I used a youtube tutorial to learn how to do them, but I can't remember which one now, I'm afraid.
The pattern I use is this (I am doing it from memory, so apologies if it doesn't make total sense - hopefully it will be OK):
Round 1 - chain 4 and join into a loop with a slip stitch.
Round 2 - chain 2 (counts as first treble), do two trebles, chain 2, three trebles, (three trebles, chain two, three trebles) three times, chain two and join to top of first chain 2 with slip stitch.
Round 3 - slip stitch into the top chains at the top of your first set of three trebles, then slip stitch into the corner chain space. Chain 2, then do 2 trebles, chain two, three trebles, do three trebles in chain space on the side of your square, then three trebles, chain 2, three trebles in the next corner space - do this all the way round.
From then on, you do the same join at the corner, slip stitches to get you to the corner space, trebles, chain 2, trebles in each corner, trebles in each chain space along the sides, and join up when you get to where you started.
Does that make sense? I hope so!