Step 4: Blocking Diode
(If you're only using one solar panel, you've actually done with soldering.)
Why do we need this? Well solar panels are great at creating power when it's sunny out. When it get's dark they try and suck power back into themselves, which then destroys them. To stop this we use a blocking diode so that power only flows in one direction.
Also, see that black bar on the diode. Always make sure you know which way it's going. You want the black bar pointing in the direction you want power to flow.