I’m about to attempt to do something similar on Deco routers, using [this](https://blog.keane.space/tp-link-deco-m5-hardware-hacking.ht...) as a base, but have never wired serial to usb manually before. My main question is how do you use a multimeter to tell which pad is which?
You first need to find ground. You can do this by visual inspection (a bit less risky), or by using your multimeter in continuity mode across external metal parts (for example the outside of the USB port)/negative pin of the power supply. Then, you can use a logic analyzer to find the serial TX pin. The RX pin is usually pretty close, and might be pulled up through a resistor to VCC.
Great article!