Since the case I chose to use is a AA battery holder it is compartmentalized inside. I needed to open it up and make room for the board, battery and wires. I used small pliers and cutters to break away the parts I didn't need.
I made sure to leave enough of the internal structure to ensure a snug fit so everything didn't bounce around when being moved.
Make sure not to damage the area around the power switch. We'll use the switch later to turn the device on and off.
Once I made room for the board I had to cut an opening for the OLED screen. I started with a small hole, that I made with a small phillips head screw driver, around where I thought the center of the screen was and then slowly made it larger and rectangular to better fit the display.
I mostly used a hobby knife going slowly and checking very often to make sure everyhing would line up.
Once I got it to the size I wanted, I used the knife moving side to side to put a bevel on each of the four sides of the opening.