There are a number of actions that will make your model building job easier:
-Placing cubes: every time you tap a semi-sphere, you'll place a cube.
-Removing cubes: retrace your steps by holding your finger against the cube you want to remove.
Selecting a different color: you'll be able to choose the cube of the color you need in the lower bar just by touching it. If the model has more than 6 colors, you can slide the color bar to the left to see the rest of the colors available.
-Moving the camera: you'll be able to change the perspective of the model by swipping your finger in any direction.
-Zoom: you'll be able to pinch to zoom in or out of the model.