Very easy, actually. Simply use a LM7808 voltage regulator IC. They're really cheap, can be had easily, and it requires all of 3 parts.
The LM7808 is a +8V 1 Amp regulator IC in a TO-220 case.
Pin 1 is the input voltage
Pin 2 is ground
Pin 3 is the output voltage (+8V)
Tie the (+) output of the solar panel to Pin 1 of the IC, and also put a .01 uf disc capactior to ground at Pin 1.
Pin 2 of the IC you would just ground.
Put a 22uf 16V electrolytic capacitor on Pins 2 and 3 of the IC, negative lead of cap goes to pin 2, positive lead to Pin 3.
Install a small heatsink on the IC as needed if it runs warm.
Hook up the (+) side of the battery you want to charge to Pin 3, and ground the (-) side of the battery. You're ready to go.
This should provide a good stable +8V source to charge your battery, up to 1 amp of current available.
Good luck!
~Cheers~