You start with creating a file like Corey said. Then you input the code. If you want to edit without reuploading, code editors are available for virtually any kind of file type.
Also, as Corey mentioned, "pages you want to be seen/files you want to be accessed/pages that require ads" belong on the www or public/html section of your File Manager.
I hope this helps! If it didn't...I didn't understand the question xD
If you need help with learning how to USE code, go
here for free tutorials.
~~Ben