Apply maskoid resist in the sky area prior to painting to retain the white.
Lay wet tissue on the dry paper and shape it into cloud formations.
Paint around and into the tissue. At this time you can use different shades of blues and grays and let them move through the wet areas.
While the sky is still wet, use a wet sponge to pull the paint in a linear pattern to simulate cloud patterns.
Adding detail and darker shades after the initial cloud formations are dry. Remove the makoid on the birds and paint them.