Use the pentool and thin brushes to draw the shapes.
Use a gradient for the sky and for the ground
step 3 of 6
Pick the some parts from the source
step 4 of 6
For the sky set the blend mode of the layer to Hard light, increase the contrast and use Image > Adjustments > Hue/Saturation to change the color. Apply a motion blur to finish.
Use the same part of the sky to give texture to the ground.
Increase the brightness of the boomerang parts and paint over in a new layer to blend them with the sky.
Use the smudge tool to remove the artifacts from the parts used.
step 5 of 6
Refine the figures and begin to draw the shadows with soft brushes in multiply mode
step 6 of 6
Improve the shadows and add some highlights also in overlay mode to finish.