Start by inserting the short 12-pin and 16-pin male headers into the Prop-Maker. We suggest using a half-size breadboard to hold the headers in place while soldering. Solder all of the header pins to the Prop-Maker.
The female headers need to be solder to the Adafruit Feather. Soldering female headers can be a little tricky, so here's a little technique: Insert the female headers onto the male pins (the ones we soldered onto the FeatherWing). Then, Install the Feather to the female headers by laying it on top of the pins, bottom side up (reference the photo). Use a PCB vise to hold the board in place while soldering.
Double check your work and ensure there are no cold solder joints. The Feather and FeatherWing should be able to be pulled apart.