As there are several types of computer cases, the steps that I am going to present here could be different for you if you want to do a similar project.
Just get an idea that, you can convert an old computer case to a smaller project box with your arbitrary size.
Step 1: Remove out everything from the case
Step 2: Remove the side walls and resize the frame
* Cut the top portion of the frame
* Attach them back together
Step 3: Cut the side walls and install the power supply
I wanted to use the PC power supply unit so I installed it back into the case. The power supply has so many outputs voltages. You may keep only those wires you need for your project and cut the rest of them.