DocBook 5.0b6 + custom schema ...

I have to create the document custom_rng.xml in the howto.xml 5.0 including 
SVG/Mathml… How to make to use this new schema? Must I modify my file docbook 
source (howto.xml) there to refer or modify the style sheet 
(_mystyle_fo.xsl)? 

Regards,
Bruno

-- 
Au lieu de subir son époque, mieux vaut la vivre.
Jacqueline de Romilly