Sit where you have an unrestricted view of the sky - outside if weather permits. Gaze at it with softly focused eyes. (Do not look directly at the sun, or you may do serious harm to your eyes.) Imagine that all thoughts, sensations, habits, doubts and worries are dissolving into the vast openness. Whenever a thought arises, or there is tension in your body, imagine that with each breath more and more space gathers around the tension, until it disappears into the boundless sky.
Anything that can help us cultivate an experience of spaciousness is all to the good. I would also recommend trying this as a visualization when a view of the sky is not accessible for you. After you've practiced this with the actual sky a few times, it will be quite possible to see the sky in your mind's eye and do the meditation that way.