Grab your 4-wire ribbon cable and find the striped wire. Leave that one alone and separate the black wire on the other edge from the ribbon cable. We'll use this single wire for our on/off switch, and the 3-wire ribbon that's left to wire our NeoPixels.
Solder a 3" wire to the middle and one of the side legs of your on/off switch (it doesn't matter which side leg). Cover with heat shrink.
Solder the two wires into the EN and G holes on your PropMaker Wing as shown. It doesn't matter which wire goes to which hole.
Stack your PropMaker Wing on top of your Feather Sense so all the headers fit into the holes. Solder this little sandwich in place.
Plug in your homemade USB cable, and use that to plug into your computer. Flick your switch on and off to make sure that's working. Also make sure you can see your CIRCUITPY drive through your homemade USB cable -- if anything's not working, now is the time to find out and fix it since these connections will become inaccessible later on.