Once you can hold the vertical flag for 10 seconds, you can work towards lowering your hips down with one or both legs in a tucked position.

1. Start in the vertical flag position, with your arms straight.
2. Bend one knee up toward your chest and slowly lower your hips.
3. Eventually you'll be able to lower yourself until your extended leg is completely perpendicular to the pole/bar you are holding on to.

Feel free to experiment with different leg positions to find what works best for you.

Remember, any modification that improves the leverage is a good way to work towards the full human flag.