When you buy a pack from Adafruit, it comes with the fully tested and assembled backpack as well as a two 14-segment dualdisplays in one of the colors we provide (say, red, yellow, blue or green). You'll need to solder the matrix onto the backpack but it's an easy task.
Prepare the header strips:
You'll need three 7-pin and a 3-pin strip of header to attach the Featherwing to your Feather. Cut the header strip to length if necessary. It will be easier to solder if you insert it into a breadboard - long pins down
Add the FeatherWing PCB:
Place the circuit board over the pins so that the short pins poke through the breakout pads
Be sure to solder all pins for reliable electrical contact.
(For tips on soldering, be sure to check out our Guide to Excellent Soldering).
Next we'll attach the LED matrices
Check the Matrices and line it up next to the FeatherWing
Be careful to NOT PUT THE DISPLAYS ON UPSIDE DOWN OR IT WONT WORK!! Check the image to make sure the 'decimal point' dots are on the bottom, matching the silkscreen.
Since there's two modules and they don't sit completely flat on the PCB you may want to use a little tape to bind the modules together to minimize any gap in between.
Flip over the kit and bend a few of the LED matrix legs out to keep the LED from slipping out while you solder
Solder the matrix in place. Note this will be a little tougher because there's not as much space. Go slow, do one pin at a time and you can clip it after each point if you need!