Use a 12-pin and 16-pin female headers on the Feather M4 Express. Insert the headers from the top of the PCB.
Use a 12-pin and 16-pin male headers on the PropMaker FeatherWing. Insert the headers from the bottom of the PCB.
Prop-Maker FeatherWing Breadboard
Use a breadboard to make soldering easier. While holding the headers in place, press the the male header pins down into the breadboard. With the PCB secured in place, solder all of the pins. Once finished, remove from the breadboard and inspect solder joints. Make sure all of the solder joints are solid.
Feather PCB Vise
With the male header pins now soldered to the FeatherWing, install the female headers to the male header pins. Make sure the Feather M4 Express is properly seated into the female headers. The two PCBs should be sandwiched together. Use a PCB stickvise to hold it in place while soldering. Solder all the pins.