A ring terminal, like this. If you want the meter to be 'always on', then use one at the first positive battery terminal and one at the last negative battery terminal. If you want it to turn on with the key, only attach the 'last negative' wire to the battery pack, and splice into one of the wires on the back of the key switch using a Scotch lock. I don't remember which wire, offhand (I think it's a blue one, but it's been a while since I've hooked one of these up). You can use a jumper wire to temporarily hook up the positive lead from the meter to the back of each wire from the key switch to figure out which one is the one you need to splice into. Hope this helps!