Note: I would make sure your Windows 8 operating system is updated because you don't want anything running the background while you attempt to switch accounts.
The first thing you have to do is get to the Your Account screen. The easiest way to do that is to open the Start Screen and type "your," which should perform a search to find an entry for Your Account.
When you reach the Accounts screen, you click the "disconnect" button.
The next screen is where you'll choose your local account user name and password. Click the Next button when you are ready.
The next page is a standard "Are you sure you want to do this" screen. Note the paragraph about files located on your SkyDrive. If everything is how you want it, click Sign out and finish.
The process will take a few moments, note that you can always go back to your Microsoft account if need be.
You now have a local account.