When you buy a pack from Adafruit, it comes with the fully tested and assembled backpack as well as a two 8x8 dual displays 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 a 12-pin and 16-pin long 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
Look for the text on the matrices. This is the 'pin 1' side, and lines up with the same side as the side with the filled in white dot on the PCB silkscreen.
There's a little 'play' in the LED matrices to make it easy to solder them in place. It isn't noticable when they're lit but if you want to make sure there's no gap between the matrices, use some tape to bind them together while soldering
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!