There are few steps to apply custom layout to a page. I have describe here how change page layout using xml. This is a process of

create custom page layout in magento

.This custom layout is show also show in admin panel In generally, page template is set under

For set temptate ,mostly using

page/empty.phtml

and Mark root page block that template is applied

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Suppose i am changing template   in checkout cart page:
 
page/2columns-left.phtml
 
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How to apply  Custom page layout handles in layout in magento shwoing here 

page/2columns-left.phtml
 
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