There are a number of different ways to dual boot, and the one you choose may depend on factors such as which operating system (OS) you currently have installed and which other OSes you want to be able to use. Two primary ways to dual boot are to have multiple partitions on one hard drive or a separate hard drive for each OS. If you choose to put several OSes on a single drive, you will need some type of bootloader and may also have to install them in a particular order. It is also possible to run some operating systems directly from removable media, so that may also be an option.
Before you begin the process of setting up a multi-booting system, you need to decide which operating systems you want to work with. It can also be helpful to consider the operating system you already have installed on your computer, since some are easier to work with than others. You will also want to decide on whether to use multiple hard drives or several different partitions. Each of these variables will change the way you alter your computer to dual boot.
If you choose to install multiple OSes on one disk, the first step is typically to partition the hard drive. You will need a separate partition for each operating system. If you are installing from scratch, you will typically be presented with an option to do this. Some OSes have partitioning systems built in, while others require that you obtain a separate program to perform this step. After you have the correct partitions, you can install your secondary OS and then modify the bootloader to present you with a choice each time you turn the computer on.
Depending on your operating system, the process of installing a secondary OS may be streamlined. In this case, you can install a new OS from within the existing system and then switch to it whenever you want. This is typically an easier process, though you are usually limited to a handful of supported secondary OSes.
The process of installing each OS on its own hard drive is typically the easiest way to dual boot. This method does not involve any partitioning, so you simply install each new operating system on a clean hard drive. Some operating systems can even be installed on secure digital (SD) cards and other types of removable media. Since each OS has its own unique drive with this method, you can dual boot by entering the basic input/output system (BIOS) each time you start the computer. This can allow you to avoid working with a bootloader at all, though you may choose to do so anyway for the sake of convenience.