The best way to simulate the look of the sky in a manga is usually by using a gradient screen tone, so that's what I did.

Here‘s my approach:
I create a new layer above my lineart, set it to multiply and fill it with the screen tone.
Then I add a layer mask on top of it and simply draw out the clouds and sun with the pencil-tool.
(The speech bubble and other stuff can be cut out by selecting them with the magic wand-tool.)