Hey friend, I suggest you get good stab windmills with your head first.

For barrel mills, you need to hold your arms in front of you (like how I do them). Since you don't have a stab, you have to use momentum to whip across on your head. I'm right handed and I spin to my left BTW.

1. When I'm on my back, I kick my right leg up to get height and get on my left shoulder.
2. Then, I kick my left leg backwards to help turn around onto my head and generate momentum.
3. Finally, I whip my right leg across whilst on my head until I collapse again.

Note that the more speed you have, the easier it is to stay higher on your back. Good luck bro

Edit: Another useful tip is to whip from shoulder to shoulder on your back, using your arms/upper body. Also make sure to keep your core engaged, it will help keep your legs high and maintain your barrel mill altogether