Order the PCBs to a PCB manufacturer, or if you're good at toner transfer on dual layer boards, build your own (I failed on that, while I usually have very good results making my own boards in the kitchen). Be careful that there are two PCB variants, one for Wifi and one for LoraWAN. Probably you want the former.

Populate the board with the few components listed. You will need soldering paste and a hot air rework station like this one. Solder the electrochemical sensor connectors (4x5 pieces) for the Winsen electrochemical sensors (ZE03-XX). Solder the ZH03A (PM) sensor connector. You'll only need 3 wires out of the original connector (Vcc, GND and Vout), and you can make two connectors out of one, by cutting it in half.

Compile the code. Use usbAsp programmer and the avrdude to write the hex and the fuse settings to the microcontroller, via the 6PIN isp connector.