XML : Using a custom TEI schema

I've built/edited a valid TEI schema using Roma. How does that custom file get attached to my XML file in order to validate the schema based off of that custom schema?

I've tried using the TEI namespace for this purpose, but the file won't validate when I do that. Help!

  <?xml version="1.0" encoding="UTF-8"?>  <TEI xmlns="custom schema link here">