As seen in Simply Knitting.
Make a foundation chain. Skip 3 chain, yarn round hook, then insert the hook under the loop of the 4th chain, yarn round hook.
Pull the yarn through the chain loop only (leaving 3 loops on hook), yarn round hook.
Pull the yarn through 2 loops only (2 loops on hook), yarn round hook. Pull the yarn through these 2 loops. Continue working treble into the next and all following chains to the end of the row.
To make the next and following rows, turn work and chain 3. This is the turning chain and counts as 1 treble. Work 1 treble into top 2 loops of 2nd stitch in previous row. Work into top of each stitch across the row.