Sketched out the first three levels.
I find drawing them out how I would like to see them in the game helps me quickly identify where I should introduce a new mechanic, where I should have the player test that mechanic and then when I should ramp up the difficulty. So for example in the level 2 sketch (middle), I want to show the player that certain things will break when the older sister hits it, I then have the player use this a few times. In the next level (last) I show the player that sometimes tall rooks in the water will break trapping them in ( and that they’ll die if they stay underwater too long) THEN I want the player to realize that sometimes THEY are the reason that the rocks can break and trap them in.
Going through all of this I can look back and see that right at the end of level 2 is a good place to show that the player breaking through things (even though they have no choice) can cause unintended effects (Like big ass rocks breaking off and hurting you!)
Once I start going into the next two or three levels (this is only a 15-25min demo after all) I will look back and see where I can add relevant information for my player or suggest things that are going to happen.