To work Zigzag stitch you work in two journeys starting first at the right.
Bring your needle up at 1, down at 2 and back up again at 1. Then take the needle down at 3 and up at 4. Begin the next repeat by taking the needle down at 3 again and back up at 4. Repeat all along your row ending with a vertical stitch.
The second journey!
Starting at the left, bring your needle up at 5, down at 6, up at 5 again and then down at 7 and then finally up at 8. Repeat doubling up each vertical stitch and crossing over each sloping stitch. This completes the Zigzag stitch!
Instead of working in the same colour for the second journey you can always use a contrasting colour, this looks good when worked as a filling as in the last photograph. More tutorials soon!