It's Called Partitioning a Drive. YOU DON'T NEED INSTALL DISK ON WINDOWS. As long as you have enough free space to shrink! How else would you set up dual booting if you didn't have one OS installed to partition a drive!
ON WINDOWS- Use Disk Management, Go to control panel or start menu, Type in Disc. You'll see something come up saying Create and Format Hard disk partitions. Or you'll see a utility called Disk Manager. From there Right click the drive you wish to partition, Select Shrink. (NOTE- You can only shrink a drive if you have extra space!) Next You'll have to decide how much space you want on your new partition. Remember to keep atleast 20GB on your current partition. AFTER YOU SHRINK-You'll see a black bar Saying Unallocated with the amount you chose to shrink. Right-click that bar, Assign A Drive Letter. Now You Done! Go to My Computer to see you new partition!
To Move it back, Format that newly created partition with no drive letter. It should say Unallocated again, Then go to your main drive(C:\) right-click, Then Expand! Basically the same process just expand instead of shrink.
ON MAC- Use Disk utility. But you'll need to reinstall the whole OS if you're doing it to you're main hard drive. But can easily be done to an external disk by going to the partition tab. Selecting the formats you want for your partitions, Then click format. Done.
You can write if you need and, Promise it's easy on Mac or Windows.