Step 6: Assembly
You are ready to assemble the device! To make it as easy as possible I used what I call 'helper wires', which are just one strand of wire that I pulled through a section of the handlebar. I used three: one from the end of the tube to the right button, one from the right to the left buttons, and a third from the left button out to the other side of the tube (see first picture).
I would tie the helper wire to the V-in and ground wires of my circuit since they were longest and put tape over the knot to reduce the chance it snagged on something. Then I would slowly pull the helper wire and push in the part of my circuit entering the handlebar so the long circuit would slowly inch its way into the handlebars. Once the helper wire pulled out the right button hole I would untie it and attach the next helper wire to the V-in and ground wires again, repeating the process until the circuit was completely through the handlebars. Be careful to not tug or push too hard if it isn't moving, it might be caught on the inside burs of one of the holes. Look into the holes and if you see wires near them take a paperclip or wire and poke them away from the opening before pulling again.
Once you have it completely through line up the buttons with their holes. You will need to find something that you can stick in the back side (opposite the hole that the button part of the pushbutton will stick through) to secure the button so it won't fall back into the handlebars when pushed. I used a tapered plastic piece (it's a plastic drywall screw anchor) I found in the basement that fit snugly in the hole and held the button well but screws or other parts should work fine.
Don't worry if you need to take out and re-insert the circuit into the handlebars a few times to readjust things, it doesn't take too long and my circuit worked fine even after several times of pushing and pulling on it.