Once you have your complete spec file modified, you process it with an XSLT processor such as Saxon or xsltproc. The stylesheet you use is template/titlepage.xsl in the DocBook XSL distribution. If your customized copy of html/titlepage.templates.xml was named mytitlepage.spec.xml, then you could process it with this command:

xsltproc  \
    --output  html/mytitlepages.xsl  \
    template/titlepage.xsl  \
    html/mytitlepage.spec.xml

The result should be html/mytitlepages.xsl. You'll find in that file a large collection of XSL templates for generating title pages.