Step 1: What You Need
A solar panel (with built in charger controller or a Solar Charging Regulator)
A car power socket
A usb car power adapter
and some wire
Step 2: Connecting the Battery
The battery is a 12V 1.3 ampHour battery The length of time the battery last very much depends on what you are doing with.
But here is quick calculation
If the raspberry pi uses 3 watts a hour then a 12v battery at 1.3 amp hours gives 15.6 watt hours
So 15.6 divide by 3 gives you 5.3 hours.
So the battery should last about 5 hours
If you want it to last long get a bigger batter or wire two up in parallel
Step 3: Connecting USB
I know it possible to make a power adapter from a 12v Battery to USB connection however this way is easier and almost as cheap.
If you would like to make your own adapter search on instructables
Step 4: Connect the Solar Panel
And now all thats left to do is power up the pi