Magento : Create a custom page in magento library

<!---- First Create One Phtml File  ----->

==> Go to app/design/forntend/default/base/tamplet/catalog/product
==> Here Copy any phtml file,
==> Paste that copied file,
==> Change the name with your Name
==> Example ::=> categoryid.phtml

<!---- Second Create One Php File  ----->

==> Go to app/code/core/mage/catalog/block/product
==> Here Copy PHP file Which name is as your phtml file which you select and copy-paste,
==> Paste that PHP copied file,
==> Change the name with Same name of PHtml file and Must "First character be a Capital"
==> Example ::=> Categoryid.php


<!---- Third Add this code in Product controller File  ----->

==> Go to app/code/core/mage/catalog/controller
==> Put belove code in this file

public function "Here your phtml file name in small letter"Action()
{

$this->loadLayout();
        $this->renderLayout();
}

==> Example ::=>

public function categoryidAction()
{

$this->loadLayout();
        $this->renderLayout();
}


<!---- Fourth Add this code in Catalog.xml File  ----->

==> Go to app/design/forntend/default/base/layout/catalog.xml
==> Put belove code in this file

<catalog_product_Your Phtml file Name>
  <reference name="content">
<block name="Your Phtml file Name" template="catalog/product/Your Phtml file Name.phtml" type="catalog/product_Your Phtml file Name" as="Your Phtml file Name"/>
  </reference>
</catalog_product_Your Phtml file Name>

==> Example ::=>

<catalog_product_categoryid>
  <reference name="content">
<block name="category" template="catalog/product/categoryid.phtml" type="catalog/product_categoryid" as="categoryid"/>
  </reference>
</catalog_product_categoryid>