Create a structure under $CATALINA_HOME/conf/Catalina corresponding to your virtual hosts, e.g.:
mkdir $CATALINA_HOME/conf/Catalina/ren mkdir $CATALINA_HOME/conf/Catalina/stimpy
Note that the ending directory name "Catalina" represents the name attribute of the Engine element as shown above.
Now, for your default webapps, add:
$CATALINA_HOME/conf/Catalina/ren/ROOT.xml $CATALINA_HOME/conf/Catalina/stimpy/ROOT.xml
If you want to use the Tomcat manager webapp for each host, you'll also need to add it here:
cd $CATALINA_HOME/conf/Catalina cp localhost/manager.xml ren/ cp localhost/manager.xml stimpy/