I suggest you visit a place called madvapes.com If this is your first attempt at modding this is a great place for you to start. You can buy ready made kits and follow the directions. They sell complete kits for box mod and variable kits with displays and all the pieces including the wire. there are also schematics to be found for building box mods all over the net. Hope this helps
Not exactly what you are wanting but gives you the idea, This is the only diagram I had with the volt meter. I have seen the one you need just didnt have it saved on my PC. This is a paralel with charger and volt meter.