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1) create your custom template: 12columns.phtml

2) create the following file and directories in /app/code/local/ :
Mage/Page/etc/config.xml

2) copy the contents of /app/code/core/Mage/Page/etc/config.xml
into our local directory:
/app/code/local/Mage/Page/etc/config.xml

add the location of your custom template in our local config.xml

This in <page> <layouts> // here </layouts> </page>


<config>
    <modules>
        <Mage_Page>
            <version>1.6.0.0</version>
        </Mage_Page>
    </modules>
    <global>
        <models>
            <page>
                <class>Mage_Page_Model</class>
            </page>
        </models>
        <blocks>
            <page>
                <class>Mage_Page_Block</class>
            </page>
        </blocks>
        <page>
            <layouts>
                <empty module="page" translate="label">
                    <label>Empty</label>
                    <template>page/empty.phtml</template>
                    <layout_handle>page_empty</layout_handle>
                </empty>
                
                <!--  this is our custom phtml file  -->
                <umi_layout module="page" translate="label">
                    <label>Umi 24/7</label>
                    <template>page/umi247.phtml</template>
                    <layout_handle>page_umi247</layout_handle>
                </umi_layout> 

                // .... additional locations

</config>




3) open /app/etc/modules/Mage_All.xml  

and change <codePool> core to local in:
...
  <Mage_Page>
      <active>true</active>
      <!--  <codePool>core</codePool>  -->
      <codePool>local</codePool>  <!-- this changes to local   -->
      <depends>
          <Mage_Core/>
      </depends>
  </Mage_Page>
...