Your plug has 2 wires.Take one wire and make a complete circut. In other words one wire goes to the heat source and the other runs through the wafer and then the heat source.
Here is a diagram https://www.gqfmfg.com/pdf/THERMOSTAT WIRING.pdf
Wire it as per the diagram I posted. The sheet metal bracket can mounted to the inside of the incubator with the adjustment rod extending through a hole to the outside of the incubator so you can adjust it. Once it's mounted and wired, loosen the wing nut that locks the threaded adjustment rod in place. By turning the threaded rod you can move the brass bellows closer or farther from the switch. By moving it closer to the switch, you lower the temperature, by moving it farther from the switch you raise the temperature. Once you're in the ballpark, very small adjustments will let you zero in on the temperature you want.