Begin with a stretched 1/8 or 1/4 sheet of paper (I used Arches 300gsm rough here). Mix up a solution of Rose Madder or Permanent rose - a blob of paint 1/2 the size of a pea mixed into about a desert spoon full of water will give the right concentration. Wet the paper and apply the wash in a horizontal band a third of the way up from the bottom. Feather out the top and bottom of the band with a Hake brush or similar large soft brush (see picture below)

Don't make your wash too dark. It will eventually be the soft pale sky behind the tree trunks.

Before starting this step, make sure your first wash has dried out completely! This might seem crazy because we are going to wet the top half of the paper with clean water again before applying the Cobalt Blue. If the first wash is even slightly damp, the next wash will disturb it and cause all sorts of strange blotches.
Apply the Cobalt wash to freshly dampened paper from the top down, gradually diluting the mixture as as you progress down the paper. By the time you reach the pink wash the Cobalt wash should be almost pure water

While the wash is still wet work from the bottom up with a dry Hake brush. Use quick, light, horizontal strokes to even out any brush marks or blemishes.

Part 2 : Foliage

While your Cobalt wash dries you can attack the foreground with a dirty yellow mixture. Stirring some water into the dry muck on the bottom of your palette, then adding a tiny amount of Indian Yellow usually produces just the right colour. (If you work with a clean palette you can mix some Alizarin and Ultramarine with the yellow)

Pay attention to the edge of the foreground where it merges with the sky. Use a damp brush to encourage the edge to feather out gradually. Make it irregular if possible (hard bits and soft bits) Don't over work it, but try to encourage it to accidentally become irregular