Cut down a tree and make 4 [[plank]].
Decide which of the tiles around the edge of your flat area is easiest to get to (its usually the one that used to be the lowest one) and, using your Mallet, right click that tile and plan a 1 tile building on that spot. (Read the guide on how to [[Guides:Build Househouse|Build a house]])
You will need a door on the '''outer''' edge of your flat space and a door leading to the '''inside''' of your 3x3 space.