Use a breadboard to help solder up the headers to the pins on the Feather M4.
Install the 12-pin and 16-pin headers onto the breadboard with proper spacing for the Feather M4.
Place the Feather M4 over the two headers.
Solder all of the pins on the Feather M4.
The Feather M4 and Servo FeatherWing are press fitted onto the socket headers on the Doubler FeatherWing.