Add method payment to existing shopping cart system

i have source code of tshirt designer software with shopping cart system, the checkout is working with paypal only. I need you replace that for 2 simple method payments: the first cash on delivery payment and the second bank transfer. Both with simple messages. thats all. You have to tell me which files you will change and what lines. All i need is the order have been processed without go to paypal. The files are php