Customize PurchFormLetter class / PO Receipts List form for My Table

I am new to Dynamics Ax 2009[:)]
In Dynamics Ax 2009, For ASN I have created a form. In header it will have the vendor name, shipment no, etc., and in lines it will have purchase order line details. (Note: ASN Lines will contain purchLines for more than one purchase order).
While clicking post button on ASN form, I have to modify the PO receipts list form and it have to show my ASN header in PO receipts list header and my ASN Lines in PO receipts List line.
I don’t know how to proceed and I don’t know which method I have to use in PurchFormLetter and where I have to use this methods.
Help me guys!!!