Instructions
Download the desired PSP firmware. Save the file to your desktop. Right-click the downloaded firmware and select "Extract" to extract the files from the compressed download. Download the Rain MMS Maker utility (see Resources).
Connect your PSP's memory stick flash card to your computer's memory card reader.
Launch Rain MMS Maker.
Select the desired firmware from the "Firmware/EBOOT" drop-down menu. Select the memory stick from the "Target Drive" drop-down menu. Select "Format Memory Stick."
Click "Make MMS" to restore the firmware to the device, creating a Magic Memory Stick for use with your PSP.