Dual/Multiple OS Installation

We often need to install multiple OS in one system, for example

1. Windows 8 & Ubuntu 12
2. Ubuntu 12 & Ubuntu 14
3. Windows 8 & Ubuntu 12 & Ubuntu 14...

We often follow the below instructions to achieve (assuming you have ONE Windows pre-installed and ONE Ubuntu pre-installed and you want to install the third OS, also Ubuntu)

1. Navigate to the BIOS, set the USB Ubuntu stick as the booting machine
2. Turn off the security boot to prevent the laptop not loading the USB (subjected to change on different BIOS)
3. Turn off the EUFI boot specifically for Windows 8 to avoid always booting to Windows instead of Ubuntu
4. Boot overriding the USB drive (subjected to change on different BIOS), if no boot overriding exists, just follow the old school boot priority function
5. Make a new partition for your Ubuntu, remember if you lack disk space, you will need to do partition resizing and  be-careful not to override the Windows partition or you may not be able to boot into Windows again!! Before installing the new OS, make sure you which partition which OS refers to.
6. After adding/resizing the partition, format it with ext4
7. You will need to provide the mounting point (I usually pick "/") and create a very small bootloader partition called "Reserved BIOS Boot Area" for GRUB's code, this is VERY IMPORTANT, grub provides a boot menu for you to navigate to different OS, we need to provide it with some space (normally as small as 1MB), after creating both partitions, you are okay to proceed to the installation processes.
8. Follow the Ubuntu install guide to finish the installation. 
9. In the final step, Ubuntu will ask you to restart the machine, just do so.
10. Luckily you will find a Linux boot menu listing all the partitions included (e.g. /dev/sda1,sda2.... etc.) but we will miss our newly installed OS partition, we need to update the grub file to reflect the changes (OS addition)
11. Boot into the pre-installed Ubuntu (the one before our third OS installation), navigate to /etc/default/grub and use text editior to edit it.
12. We only interest in GRUB_DEFAULT, this controls which OS the default OS should boot in, choose the one you want and save the setting
13. Remember after updating the file, you will need to UPDATE by sudo update-grub , if not the newly installed OS will not appear in the grub menu
14. Reboot the machine and you will see the newly installed OS appeared.

Reference:

1. http://askubuntu.com/questions/16042/how-to-get-to-the-grub-menu-at-boot-time
2. http://askubuntu.com/questions/343268/how-to-use-manual-partitioning-during-installation