The Mayfly has an ADS1115 16-bit A/D converter on it. To use it, you can use the Adafruit ADS1x15 library and follow the tutorial on this page: https://learn.adafruit.com/adafruit-4-channel-adc-breakouts?view=all
The library comes with examples for single-ended and differential measurements. So you can follow the differential example to take a reading with the Mayfly. The ADS1115 on the Mayfly is at the default I2C address of 0x48. Using the default gain of 2/3, you’ll have a resolution of 0.1875mV per bit. So if you want 0.1mV resolution, you need to switch the gain to 2 so you’ll get 0.0625mV per bit. But that also limits the range of the maximum input voltage, so be careful not to exceed it. Exact details about the resolution and gain settings can be found in the example sketch that comes with the library.