The first thing to do before starting to charge with a solar panel is to install the large filtering capacitor. This capacitor is necessary to stabilize the panel a little, and since we're going to charge with a lot of current, the capacitor needs to be pretty big. There is a spot on the PCB for the capacitor, you can of course install it directly there by soldering the wires of the capacitor into the pads.
If you're low on space, you can solder wires to the capacitor and then install it a little farther away. Just remember to use heatshrink to keep the capacitor legs from shorting. If you want you can bend the capacitor over a bit as well, but dont have it touch the hot charging chip (the black square in the middle of the PCB)!
Next you will have to connect up your solar panel. Panels sometimes come 'bare' (no wires), with just wires, or with a connector of some sort. You may need to splice a connector onto the panel to match the 5.5/2.1mm (or 4mm on older charger) DC jack on the adapter.