It's easy to install dual OS, but if you install Windows after Ubuntu, Grub will be affected. Grub is a boot-loader for Linux base systems. You can follow the above steps or you can do just the following:
- Make space for your Windows from Ubuntu. (Use Disk Utility tools from ubuntu)
- Install Windows on freed space.
- After installing, login to windows
To fix this you can install 'EasyBCD' in Windows.
Download it here
Follow these steps to restore GRUB when after installing EasyBCD:
- Launch the program and select ADD NEW ENTRY from the EasyBCD Toolbox
- Select the 'Linux/BSD' from the operating systems column
- Choose GRUB (Legacy) under type and click on the ADD ENTRY icon
- Choose YES to the restart prompt
GRUB will be displayed after the restart and will detect the Ubuntu partition for you to be able to boot into Ubuntu.