Please watch this video if you are having trouble making the Simple Blocks in onshape. You may pause, rewind, skip ahead and watch this video as much as you need. This video is a silent video of me making all 6 of the blocks. This is not the only way to make them, but this is how I went about making each block. 

Even though this should be a refresher for you I have included this video again for what to do when you finish your 3D model in Onshape. In the Simple Blocks video for time sake I did not do these steps, but you need to in order to earn full credit.