Make your canvas size 800 x 600. Adjust your workspace so you see it all at 100%.
Make the canvas color blue. Do not use the Paint Bucket! Click off the canvas with black arrow and click on the color square in
Properties (or go to Modify>Canvas>Canvas Color and set the color there).
Make sure you have a bitmap layer so you can draw...
Set the Brush color to white, Unnatural>Viscous Alien Paint...
Draw a few clouds in your sky. Name your bitmap layer "clouds".
Insert a new bitmap layer.
Set the Brush type to Airbrush>Basic, yellow, size 100 - 200.
Click a few times in the middle of your stage, until you get a sun you're happy with.
Select your black arrow tool, then you can move the clouds where you want.
Also move the "sun" layer under the "clouds" layer.