The last step is to add your new file of templates to your customization layer. You can do that most easily with an xsl:include statement:

<?xml version='1.0'?> 
<xsl:stylesheet  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"  
      version="1.0"> 
<xsl:import href="docbook.xsl"/> 
<xsl:include href="mytitlepages.xsl"/>
</xsl:stylesheet>  

This href example assumes you put your customization layer and new title page template file with the DocBook distribution files. You may have to adjust the paths. Once included in this way, your new title page templates will override the default templates in DocBook XSL. Note that if you have customization layers for both chunked and non-chunked output, you'll need to add the include statement to both customization layers. But by keeping the new templates in a separate file, you can easily regenerate it without needing to re-edit other files.