Step 6: Wire it all
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 the circuit with the diode. A diode will only allow the current through in one direction. Which one confuses me each time anew, so you will have to figure it out. (Sorry :-) )
Attach the "+" to the one wire to the buckle switch and the "switch" wire to the other.
I used heat-shrink tubing around all connections in the wiring and around the diode.
I used some sort of silicone tape that doesn't actually have glue to neaten up the wires once all works. The tape worked well enough on larger diameters, e.g. when looped through the helmet, but it did not work so well around just wires.