Try this: Step-By-Step: Partition Your Hard Drive | PCWorld
This should give the step by step instructions you need.
For a specific Windows 8 one try this: Windows 8 disk management: extend, shrink, resize volume
Here are the steps as layed out on first link:
Check, optimize, and back up your drive(s)
First, select Start, Programs, Accessories, System Tools, ScanDisk, click Thorough, and check the Automatically fix errors box. Then click Start.
Next, defragment each drive by selecting Start, Programs, Accessories, System Tools, Disk Defragmenter.
To use Windows 9 x's or Me's built-in backup utility, select Start, Programs, Accessories, System Tools, Backup and follow the directions. (If the utility isn't there, then for Windows 9 x, check Start, Settings, Control Panel, Add/Remove Programs, Windows Setup, Accessories, and for Windows Me, run Msbexp.exe from the Addons/MSBackup folder on the Me CD.) In Windows XP Professional, select Start, Control Panel, Performance and Maintenance, Back up your data.
Plan for partitions
Before you run the partitioning software, decide how you want to organize your hard drive(s). Consider the size of the drive, whether you want to simply segregate data from the operating system and applications or create a more complex structure, and whether you want to run more than one OS. Read your partitioning software's manual for suggestions. Major partitioning utilities also have wizards that lead you through the process.
Run the software
Once you've decided how to set up your partitions, follow the directions to install and run the partitioning software. Although the software won't let you make incorrect choices, you should check and double-check the new partitions to make sure you've allocated all the drive's space. Creating and moving partitions may take 30 minutes or more, depending on how full your drive is. The software will also format the new partitions for you.
Reboot and reorganize
Once the partitions are created, reboot your PC to complete the process. You can then move your data around or install a new operating system. If you plan to store all your data in one of the new partitions, you'll have to tell your applications where to save files in the future. Usually, you can set this from the Preferences menu or File Locations tab in each application.
If you have problems accessing or using the new partitions, use the emergency disks that came with the partitioning software to double-check that the partitions were created and formatted. If they still don't work, then it's time to contact tech support.