Cut out the shapes:

11x4" Blue rectangle for sky

3x5, 3x4, 3x3 color rectangles for trees

Cut rectangles into tree forms:

ovals, egg-shapes, circles, bean-shapes, etc.

Draw the tree details

Start with the center line

Add branches, acorns, etc.

Glue the larger shapes first

Have them overlap

Add smaller shapes

Extend the tree trunks in the foreground.

Add the sun.

Cut off the sides of the collage

Mat it.