Draw a sketch by clicking on the rectangle draw tool in the tools menu and then clicking and dragging on the viewport. Finally close the sketch to turn the drawn lines into a 3D face.
Use the middle click to orbit around that face and inspect it. Position the camera so it points at the face slightly sideways.
Now turn that face into a cube by extruding it - select the Push/Pull tool, click on the face, move the mouse until the face starts extruding, then type 1mm and press the enter key.
The resulting object is not quite square, so let's go back and edit its sketch by double clicking on the Sketch step inside the browser menu. Now select one of the horizontal and one of the vertical lines, using the select tool, by clicking on the first one, then holding the ctrl key and clicking on the second one. Click on the equal constraint to make both of the lines the same length. Now select the vertical of the two lines, press the k key, followed by the i key to set a height constraint and enter 1mm when prompted for a value. Close the sketch again to see the object is finally a perfect cube.
Press the b key to select the color bucket tool and paint some of the faces with different colors.
Create another instance of the cube's body by double clicking on the "Bodies" tab in the browser menu - this will initiate the placing of another cube inside the viewport - press the left arrow key to constrain the movement to the Z axis and click to finally place the new cube in line with the old one.
The whole scene can now be saved for later via the system menu.