HERE pretty much.
Process is exactly the same as the PS Jailbreak...
How to downgrade firmware 3.42/3.50 or above to firmware 3.41 with E3 Card Reader :
1. Insert E3 Card Reader device that has been reprogrammed as downgrader into your console
2. Turn on the console using the same boot method as all other jailbreaks: Press power then immediately press eject. Your console will turn on and E3 Card Reader will light up red LED followed by blue LED
3. Turn off system via menu
4. Remove E3 Card Reader dongle and turn on console to make sure you are in Factory Service Mode. You will see a huge red box on the screen saying Factory Service Mode.
5. Load any blank FAT32 USB flash drive device with 2 files in the root directory:
1. Modified 3.41 PUP
2. Lv2Diag.self (File 1)
6. These must be the only files on the USB key and must be placed in the main (root) directory.
7. Insert flash drive into the last USB port to the right — the first USB port in from the right side if you’re facing the console. Turn the PS3 on and you will see black screen but the PS3 HDD light will flash.
8. The process takes 3-4 minutes for the system to install the old firmware. Towards the end, the power button on the console will start flashing green and then your console will power off.
9. Remove the flash drive and turn on the console, you will still be in Factory Service Mode. Please verify you have downgraded successfully to version 3.41 by going to "System Information".
10. (Optional) – If you wish to downgrade to any older firmware you can follow step 4, 5, 6 using any regular firmware PUP. (You do not need to use modified custom firmware now that you are on 3.41).
11. Insert USB flash drive back into your PC and delete the PUP and Lv2Diag.self file (File 1) and copy the other Lv2Diag.self (File 2) to the root of the flash drive.
12. Insert USB flash drive back into the same USB port on the PS3 and power on your PS3. After 10 seconds it will power off.
13. Remove USB flash drive and turn on your console again. You will be out of factory service mode and back in retail mode with your chosen firmware version installed. You will need to go through setting up settings configuration.
To go back to E3 Card Reader jailbreak device, just drag back the normal .bin files.