Try searching under "top-down design". There are a number of different ways to approach this type of work. Top-down design usually means that you design the case parts in an assembly with the circuit board. Create an assembly and place the circuit board in the orientation that you want. Start a new part, say the top case, in the assembly (Insert Component/New Part, pick a plane or face for your first sketch.) Now create features in your new part, referencing the geometry in the circuit board.
I would probably use a variant of this approach called the "master model". I would make a multi-body part that includes the upper and lower cases. I would add all the details that interact between the upper and lower case, like screw bosses or snap details. Then split the master part into the new upper and lower case parts and add remaining details that aren't shared.
You could use those two terms to search in the forums in SW Help or more generally (say on You Tube) for tutorials.