Continue filling in the sky but push lighter and lighter as you go towards the middle line on your paper.
(ART term: That middle line is called the Horizon Line.)
Colour a thick dark blue stripe at the bottom of your paper to start the sky reflection!
(Did you know: Everything under the Horizon Line in your picture is reflection in the lake.)
Fill in the lake moving upwards lighter and lighter the same way you did the sky!
Add a light swoosh between your clouds!
(If you coloured cloud 3, of course. If you didn't, then colour lightly under cloud 2, all the way to the edge of the paper.)
Do the same thing upside down!
Fill in the rest of the sky & lake with VERY LIGHT side-to-side strokes.
Add white (or light blue) on top of the blue parts of your sky... blending the blue & white together!
Same thing upside-down for your reflection in the lake!
Keep filling in the white & blending with the blue until it looks like a beautiful sky!
Same in the Lake!
Use your white to swirl the edges of the clouds in to the sky a little! (Make them, fluffier!)