First go the following web address to get started.
http://www.fafsa.ed.gov/
Next you will hit the option 3 which states FAFSA followup. You must have already filled an application out prior to being able to add more school codes so please keep that in mind.
At the bottom of the screen you should see add or delete a school. Hit this next in order to add the school code.
Next you must select the school year in which you did the FAFSA. Than you will need to hit the next button.
Fill out all the information needed such as name, social security number as well as your FAFSA pin number. Than hit the next button again.
Finally you will be directed to the page to add or delete a school code. Input any new school codes you want to add and delete any you are no longer needing. Thats it you have added a school code to your FAFSA application.