If you already submitted your tax return but realized that you forgot to include something, and you need to now file for an extension how do you go about doing this?

You do not need to file an extension for your return because it has already been filed.  

If you forgot to include something, you will need to Amend your Tax Return by filing Form 1040-X.  

You can click here for instructions on Amending your tax return: How to Change a Tax Return You Already Filed.