I don't know if this is the simplest, this is what I did.
SDS011 with USB interface
I found some python code online and modified it to read the sensor every 5 minutes and send the data out via MQTT. This is running on a old Raspberry Pi 2 in my backyard.
I have another Raspbery PI running mosquitto as the MQTT broker, though this easily could be the same machine as is running Weewx.
On the machine running Weewx, I wrote another tiny piece of python that subscribes to the MQTT messages and when it gets one, it writes the data to a file.
I modified my Weewx driver to look for that file and put the data in the PM10 and PM2_5 fields. Then, everything else in Weewx is normal.
The code for that talks to the sensor is in the aq-sensor folder.
Like I said, I don't know if this is the simplest, but it's working for me. There's probably a more official way to do it with MQTT, that doesn't require mucking with the driver. I had already modified the heck out of mine, so it was the easiest for me.