how to create customized payment?

[templario]

What changes do I need to create a customized payment

[AH]

You need to read the forums for plugin development depending on what your needs are you may be able to use an existing plugin payment type.

[julianajones]

The following is a list of all methods that must be implemented in a payment module's class file.
string show_configuration( void )
Shows the configuration form for this payment module in the payment method form.
boolean has_configuration( void )
returns true if the payment module can be configured, false if not
boolean configfile_writeable( void )
returns true if the configuration file for that payment module is writeable, false if not
boolean configfile_readable( void )
returns true if the configuration file for that payment module is readable, false if not
void write_configuration( Array )
stores all configuration values for this payment module in the configuration file.
boolean process_payment(String $order_number, Float $order_total, Array &$d)
This is the main function for all payment modules that use direct connections to a payment gateway (like authorize.net or eWay XML). This is the place, where the payment details are validated and captured on success. Returns true on sucess, false on failure.
float get_payment_rate(Float $subtotal)
This is the function to calculate the fee / discount for this special payment module (so you can calculate a fee, depending on the order total amount).
If you are about to change the Name of Configuration Variables: don't forget to change it in both functions: show_configuration and write_configuration!
Examples
Credit Card Processor: Authorize.net
This example shows the payment processing class for authorize.net. It demonstrates how to show a configuration form, save the configuration and process the payment over the internet.
Authorize.net Payment Class
HTML Form-based: PayPal
This example shows the payment class for PayPal. PayPal processes the transaction on its own sites and sends a notification to the shop about the status of the payment.

That’s why an additional file is needed to receive this notification: Invalid link [notifiy.php]. It’s an implementation for PayPal’s IPN (Instant Payment Notification).
Here's a link to the latest version of the Invalid link [PayPal Payment Class].