Create a new assembly document.
Close the Begin Assembly PropertyManager if it appears.
Open these three parts you just created: the left button plus the upper and lower housings.
The right button is already open.
Click Tile Horizontally (Standard toolbar) so that all five documents are visible.
In the LeftButton document, drag the part name, LeftButton, from the top of the FeatureManager design tree and drop it onto the Assem name at the top of the assembly document's FeatureManager design tree.
Dragging this way aligns all the documents properly to the new assembly document origin.
Repeat the process for the right button, upper housing, and lower housing parts.
Save the assembly document as AssemblyBase.sldasm.