First, check this for a list of compatible devices. Choose your device and follow the instructions given.
Second, download Backup Manager and put it on the root of a flash drive (or other memory device).
Third, follow these steps (taken from here):
Once you're ready, turn the switch off in the back of your PlayStation 3.
Make sure there are no other USB devices plugged into the PS3 (USB Stick, Sixaxis Controller, etc.).
Plug in your device and then switch the back power button on.
Once you have that done, quickly press the power button, then hit eject as fast as you can.
If it takes longer than usual to boot up, it was a success.
The PS3 should boot up and you will see two new options in the games portion of the XMB (cross media bar).
You can now safely disconnect your device and return it to normal operation mode while enjoying this exploit.
To use Backup Manager, simply plug in the flash drive you put it on, look for Backup Manager in the Games section, and install it.
The jailbreak is temporary; simply put, it goes away when the system is turned off, and unsigned applications/packages will no longer be usable. In order to jailbreak it again, simply repeat step three.
I hope this was simple enough for most of you to understand. I'll try to keep this updated if simpler methods are discovered.