hw to modify medruck & invoice

hii folks,

plz do let me knw,

1)hw to modified Print program and layout set Purchase Order n if i want to insert company header, for the supplier address, delivery address.hw to achieve this things.(plz tell me step by step procedure with examples)

2)hw to modified layout set for INVOICE form based on standard layout set RVINVOICE01.(plz tell me step by step procedure with examples).

for answers points will be rewarded.

regards

sweta.