Power Supply Assembly:
Look at the back side of the power supply.
That's the side where you see the power plug socket and quite often a switch just above or below that socket.
Find the screw holes in the outer edge of the supply case, you will usually find one in each corner except one. That one will be just a little above the lower left corner and is set off so that you will install it right the first time. (lol)
If you do not see the arrangement as shown in the photo below then either the supply or the case is most likely what's called a proprietary part.
A proprietary part is one that's made to manufacturer's specifications and will not fit into standard generic case assemblies. Or if its a proprietary case then the usual generic standard parts most likely wont fit into the case.
Either way my advice here would be to replace the power supply or case so that you are using standard parts for your assembly.
Line up the screw holes in the power supply with the ones in the case and secure the power supply with the screws provided with the case or that came with the power supply.
If screws came with the power supply you bought then it's best to use them as some cases use a longer screw that will go deep into the power supply and maybe touch something it shouldn't.
You don't need to cinch the screws down just make them good and snug.
That completes the power supply assembly.
If your case came with fans installed and you don't need to install more then skip down to installing the motherboard assembly in the chassis.
Otherwise install all the fans now.