Creating 3D objects

I started out by creating a rectangle with the top view. 

There I could pin point the hollow circles. 

Now it was about finding the the rectangle for the second wall of the object. 

EXTRUDE! 

In order to make sense of the holes. [booleandifference] was used. I noticed there was a difference so 

highlighting both circle curves you extrude a second curve but with a depth 5 points less than the surface. 

Now selecting the next piece that will rise. Creating a 

I have Extruded! and looking for the dimension to which I will take [Booleandifference] 

Creating points for the decreasing. 

[Booleandifference]

Creating those [Chamfer]s

COMPLETED!