You will need:
A base coat
A sky blue nail varnish - for this I used Barry M nail paint in blueberry
A white nail varnish
A top coat
A large dotting tool ( if you don't have any dotting tools, you can get them really cheap from eBay or amazon or alternatively use the end if a kirby grip)
First apply the base coat, and once it has dried, apply the blue nail varnish to all of your nails.
Then once the nail varnish has dried take your dotting tool and dip it into the white nail varnish. To make up the clouds, create white dots that are touching each other.
Do this with about 5 or 6 dots until a cloud is formed.
Create two or three clouds on each nail until your nails look like a lovely bright sky with perfectly formed white clouds.
Go round each nail with a cotton bud to neaten up the edges and make it look very tidy.