I'll be using boot manager application in this guide. It's not my app, I didn't develop it and I didn't help in developing it in any way.
The application doesn't touch you internal memory at all. It just makes .img files for the system, data, cashe and boot. It also makes a .android_secure folder.
So any change made will be in the SDcard not in the Internal memory.
That's enough talking, now here are the steps for installing a ROM and booting to it. But before you do anything, make a Nandroid backup (just in case if something goes wrong)
1- Open market (play store) and download Boot manager (lite or pro, but I will be using the lite version in this guide)
2- Open the app. It will make the first time configuration in which it will download about 3~4 MBs.
3- [IMPORTANT !!] Setup your phone ROM. This creates a boot image file for your current ROM so you can boot back again to your original ROM.
4- After that, swipe to go to the first ROM slot. Press install zip button, choose your ROM's zip file location, then check the three boxes (system, data & cashe). Then press okay, then choose which file system you will be using.
Now it's creating files for your ROM inside /SDcard/BootManager/rom1.
You can see the progress in the notification dropdown menu. Try not to use your phone while installing because it will be very slow and laggy.
5- Now it's time to install gapps or any other mod you want to install. Do the same steps as the ROM installation but this time don't check any box. If your ROM doesn't include gapps then you MUST install it as boot manager needs to check your license.
6- Now you can boot to the installed ROM. JUST press boot ROM
The booting process will probably take more time than usual. That's normal don't worry about it.
OK now you're on a SDcard ROM and you want to go back to your phone (original) ROM. Just open boot manager, swipe to phone ROM then press boot ROM. It's that simple
Some problems I had:
When I wanted to go back to my phone ROM, it doesn't boot, it just open the recovery and it doesn't want to reboot properly.
Make an advanced restore for the system from the recovery then boot. If it didn't work then make another advanced restore for boot. That's the only solution I have found till now.
Tips and Tricks:
1- If the performance is slow, overclock the device.
2- Use SDcard booster, it's found on the play store.
3- Also use swapper, also found on play store.
4- You can also edit some details, as the ROM and kernel name and screenshots.
5- If you were using the lite version you WILL find the paid one in the SDcard ROMs. But don't be happy as it won't open. Just uninstall it and then install the lite version again from the play store.
6- If you are like me, in a country in which billing in android market is disabled or you just don't want to pay for the pro version, do the following:
Although the lite version has only one rom slot but it creates 5 folders rom1, rom2,.... etc
And it only creates the files in the rom1 folder
Let's say for example you are running CM7 as your daily driver and you want rom1 to be LeWaOS and Rom2 to be CM9 KANG.
It's quite simple, first install CM9 normally, it will be installed to Rom1 folder. Then copy every file in that folder to rom2 folder and delete the contents of .android_secure folder in rom1, then install LeWaOS normally.
If you want to boot into LeWaOS u can boot normally using boot manager. If you want cm9 then copy the files inside Rom2 folder back to Rom1 then boot normally through boot manager