Choosing the Right Board
The larger board allows for a relay to be mounted on the board itself. This is the configuration that works best for a power supply, such as an ATX, that has a "PS_ON" (or switch on) circuit. The larger board could also be used for a single relay, of sufficient amperage, that provides power via a single bus. However, DO NOT wire the "PS_ON" connection on the PCB to anything but a PS_ON circuit!
Choosing the Mounting Location
Two examples of mounting are shown here though others are certainly possible. The major question will be one of where the two sensors need to be located relative to where the PCB can be mounted. If the PCB can be mounted where the sensors need to be located then the sensors can be surface mounted on the PCB. If not then leads will be required to connect the sensors to the PCB. Likewise the monitors LEDs, reset, and bypass switch can all be surface mounted on the PCB or run remotely depending on placement of the PCB.
Surface Mount or Headers
Two strips of breakable headers can be used to provide mounts for most of the components and connection points on the PCB. This can allow for easy removal of the Arduino Nano as one example.
There are solder points on the PCB that expose a number of available pins (including power) on the Arduino Nano. These could be used for a variety of purposes such as connecting additional sensors and/or relays.
Actual assembly is straightforward. Either solder the components (or leads) to the board in the places marked or solder on the appropriate headers! Header strips can be easy broken into the proper lengths using a small wire cutter positioned on top of the pin where a cut needs to be made. A pin is lost with each cut. Edges can be sanded to make a smooth end to the header.
Polarity and/or Pin Positioning
The short lead on the LED is the ground. The PCB shows how the Nano should be mounted as it does for all of the components and connections. The reset button should be mounted as shown by the picture to the right.
The larger PCB provides a place where a relay can be mounted. Small cable ties are provided for this purpose. The larger PCB also allows for a bypass switch that mechanically closes the "PS_ON" circuit assuming that circuit is connected to the PCB. Do NOT connect anything but a PS_ON circuit to the PCB.