Attach all "-" wires to each other, i.e. from battery to solar cells. Attach the "+" from the solar panel to the "+" from the batteries but protect ...
Step 4: Pre-wire the solar cells
In the yard lights they use one solar cell for one battery. In the bike lights there are two batteries in series. There are two bike lights. So, I use two pairs of two solar cells in series to charge the four batteries in the two bike lights. My solar cells have their terminals on the back side, so I needed to wire them before gluing them down. Decide how you will lay out the solar cells on the helmet and wire two pairs where a solar cell "+" is connected to a solar cell "-". Also attach wires to the lone "+" and "-" terminals.
I forgot to take a picture of the back side of the solar cells before mounting them, but it's pretty straight forward.