Be aware of the circle made by the horizon and yourself at its centre. Be aware of your own centre. Practice a constant awareness of both your centre and the horizon, you don’t need to focus on either, just know they’re there. You are always in the centre of the circle no matter where you are or what you are doing.
If you have trouble imagining the entire horizon, start by visualising two points opposite each other on the horizon, one to your left and one to your right. Focus on them both at the same time. Relaxing your gaze and using wide angle peripheral vision is helpful.
Your centre can be anywhere in your body. If the Tanden, Hara or Dantian mean something to you, then use that.
Breathe. Relax. Enjoy.