In the videos below, I will be demonstrating how to correctly dimension this object in Onshape. 

This video talks about dimension and how information about the size of an object will be displayed. 

This video discusses weak and strong dimension and what happens if you try to over-constrain a part. 

Take a few minutes to practice placing dimensions. In particular, make sure you're comfortable setting dimension for angles and diagonal lines. 

This video talks about how to change your dimensions in sketcher. Remember to start your drawing at the origin point, and to dimension your first line prior to drawing the rest of the lines. 

If you forget to dimension your first line when you draw it, you can still scale the the entire sketch just by removing all dimensions except one and then adjusting that number. If you have multiple things dimensioned each item must be changed individually, but if you only have one thing dimensioned, Onshape will instead scale your drawing. Watch this video to see what I mean. 

This video demonstrated how to specify the depth of your extrusions.