Before installing the microcontroller, solder each of the four jumpers on the top side of the PCB. Don’t forget this step, as it’s very difficult to solder them after installing the microcontroller.
The jumpers are located on the top side of the PCB, underneath where the microcontroller is to be installed.
Even if you're not using an OLED display, there's no harm in soldering the jumpers. Feel free to do solder them, so it's easy to add displays later on if you change your mind.
To solder a jumper, place the tip of your soldering iron between the two pads, so you'll heat both of them at the same time. Then, slowly feed a bit of solder onto the tip of the iron. Because both pads will be heated, a joint will form. There's no need to hold your soldering iron on the pads for long: it often only has to take a few seconds.
Do not solder any other jumper than the four jumpers on the top side of the PCB. Soldering a jumper on the bottom side of the PCB causes the OLED display to malfunction or even be damaged. In the event that you have accidentally soldered a jumper on the bottom side, remove any solder with a solder sucker tool, and remove any trace amounts by cutting a line between the two jumper pads affected with a hobby knife.