[Optimus One, P500 Original Android Development] Tasssadar:
OK, here goes guide for average user. This is still highly experimental feature, keep that in mind. Backup, backup backups and backup them to PC, read whole guide first, I am not responsible for your bricked phone etc.

GUIDE:

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1. Requirements

Ext4/Ext2/Ext3 partition on SDcard big enough to hold all files from /system and /data, which is from 170mb to 400mb (depends on ROM and apps installed), so I think 512mb partition is minimum (if its 90% or so free).

Modified recovery. Only AmonRa and CWM version is available at this time, TeamWin is coming "soon". You can find flashable zip at the bottom of this post, flash it before you proceed to next move. If something goes wrong, you can restore your recovery with zips from here.

Kernel with ext2/3/4 support, which is basically every but stock

Both ROMs you wanna use must be compatible with same kernel. See Limitations in previous post

2. MultiROM instalation

- Via MultiROM Mgr application

I recommend you do nandroid backup before installing via this app.

Download MultiROM Manager (link at the bottom of this), tap to "Tap to install via this app" and wait. As simple as that. Mgr will also install recovery for you (It will ask you which one, you can select AmonRa, CWM and none). App can check & install update, too.​

- Via recovery

Just download multirom.zip at the bottom of the post, make nandroid backup and flash it. Your current ROM will be the one in internal memory, we will setup the one on the SDCard in next move.

Now, try to reboot the phone (boot will take little longer, because it has to mount ext partition). If it boots up correctly, then its OK - your ROM is supported. If not, then restore your nandroid backup and post your ROM name here, maybe I'll try to make it working.​

3. Setup ROM on the sdcard

Make nandroid backup (I am not kidding, you will need this one), flash ROM which you wanna to have on SDcard, do the first boot (because it will create dalvik-cache, and you dont wanna do that when its already on sdcard) and reboot to recovery again.

Go to "MultiROM options" menu and select "Create from current ROM". It will do everything for you (it takes time, though). ROM on the sdcard is placed in /sd-ext/multirom/rom/, backups are in /sd-ext/multirom/backup/.

Restore nandroid backup from beggining of this step and reboot. Wait until Boot manager shows up, and select SDcard as boot location. ROM from sdcard should boot. If your ROM from internal memory boots or if its stuck, something got wrong and you should restore from backup.

4. MultiROM recovery menu description

Active/deactive state is for recovery only, you can boot ROMs from backup in boot manager - that means its N-times boot rather than dual-boot.

When not active(boot from internal memory):

Activate (move from backup) - when you already have backup, this will move it to active folder (fast).

Activate (copy from backup) - same as above, but it copies it instead of moving

Create from current ROM - pretty self-explanatory

Create from ZIP file - see http://xdaforums.com/showthread.php?p=22194117#post22194117

When active(boot from sdcard):

Deactivate (move to backup) - Moves current ROM to backup folder, so next boot will be from internal memory

Backup - Creates backup of current ROM

Erase current ROM - Deletes current rom, next boot will be from internal memory

Copy modules from int mem - Copies modules from ROM in internal memory to SDcard. Try this if WiFi does not work in SDcard ROM.

Flash ZIP - flash ZIP file to ROM on SD card, for example gapps.

5. Config file:

It is recommended to use Multirom Manager(see bottom of this post) to change config file.

File itself is placed on sdcard and its name is multirom.txt. Description of its contents is in this post(post is updated if something changes): http://xdaforums.com/showthread.php?p=20045183#post20045183

6. Key bindings:

Since v11, you also can control MultiROM via touchscreen.

Anywhere: Hold Power button to shut down the phone

While on main screen:

VolUp/Down - list between actions

Menu - Confirm action

Back - Reboot to recovery

Power button - Sleep mode

While playing tetris

VolUp - return to main screen

VolDown - pause

Menu - move the piece to left

Home - hard-drop

Back - rotate piece

Search - move the piece to right

FAQ:

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Q: It is slow!

A: Its sdcard. It just is slower than nand memory.

Q: It does not work with my ROM

A: Post your ROM combination here, maybe I'll see to it.

Q: When do I need to reflash?

A: When you flash new ROM or do something which rewrites boot partition. Most (if not all) kernels are using Koush's AnyKernel, so they should not cause any problems.

Changelog & Download:

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(Always flash newest version)

Uninstaller: http://www.mediafire.com/?3eco51f2t0at77o (info here)

Changelog AND DOWNLOAD LINKS have moved to github: http://tasssadar.github.com/multirom/

Recoveries

AmonRa - based on 3.06: http://www.mediafire.com/?ld9g81igvze7na0 (9th March 2012 21:57)

AmonRa - based on 2.2.1: http://www.multiupload.com/7Y5HHNEPOT (20th October 2011 22:29)

CWM: http://www.mediafire.com/?6rfw0qbrddzq5d8 (9th March 2012 21:57)

TWRP: http://www.mediafire.com/download.php?o8kphxmfk9l01tl (21st October 2012 23:49)

MultiROM Manager app

http://xdaforums.com/showthread.php?p=20610052#post20610052

(Always use newest version)

v24 - http://www.mediafire.com/download.php?1xy52tamzepibrx (TWRP support)

v21 - http://www.mediafire.com/?u9c3bmfbtf57dtm (Add charger settings)

v18 - http://www.mediafire.com/?g1lpsp66jc1cb1b (Add boot location settings)

v17 - http://www.mediafire.com/?l1li51sseaisz5j (Add brightness setting)

v15 - http://www.multiupload.com/B65WPFMFXP (Show & reset tetris high score)

v14 - http://www.multiupload.com/FWB5UBSMO1 (Updater now also check for app update, some minor fixes)

v13 - http://www.multiupload.com/NYYBX9CAFD (Update translations, add support for optimus V - http://xdaforums.com/showthread.php?p=20657898#post20657898)

v11 - http://www.multiupload.com/1L3294ZAU7 (Fix mount issues on some roms, add menu to main screen, add link to xda)

v10 - http://www.multiupload.com/B3AVSFKDOW (Create /sd-ext/multirom folder on install)

v9 - http://www.multiupload.com/WW7TUNP7RY (read http://xdaforums.com/showthread.php?p=20624306#post20624306)

v8 - http://www.multiupload.com/6OD7RQ74OE

v7 - http://www.multiupload.com/DJKMUGIVJ0