We'll use a FeatherWing Doubler to connect the Adalogger FeatherWing to the Feather nRF52840. This provides a nice, accessible layout.
The FeatherWing Doubler provides the same connections between the pins of the Adalogger and the Feather as if they were stacked in the more typical arrangement, so all data, power, and ground connections will work as normal.
You may, of course, choose a different arrangement, such as stacking the boards if you like.
Solder male headers to the underside of both the Feather nRF52840 and the Adalogger FeatherWing.
This guide has good tips on soldering in the headers.
You will also need to install the battery (with the + side up) into the Adalogger's battery holder -- this powers the real-time clock so it is accurate even when the Feather is powered down.
You can also insert the SD card at this point.
Solder female socket headers to the top side of the Feather Doubler.
Now, you can insert the Adalogger and the Feather into the Doubler.
This is now ready for coding!