In CRM/Core/SelectValues.php:
- add new payment processor to function &paymentProcessor, around line 597

In CRM/Core/Payment/Config.php:
- around line 155: switch ( $config->paymentProcessor ) {
  - add info for the payment processor.  Use the same name as in the previous
    step.  At the very least, you will need to set $config->paymentFile and
    $config->paymentBillingMode
  - The paymentFile should start with 'Payment_', indicating that it's in the
    Payment subdirectory of the relevant CiviCRM modules

Decide what fields need to be displayed for the payment processor.  Compare
to existing fields.

In templates/CRM/Admin/Form/Setting/Payment.tpl:
- If necessary, if the new payment processor requires fields different from
  any of the currently available sets, add a div with the id representing 
  the new payment processor.  The format is like:
    <div id="processor">
        <dl>
        <dt>{$form.[field name].label}</dt><dd>{$form.[field name].html}</dd>
        <dt>&nbsp</dt><dd class="description">{ts}[field description]{/ts}</dd>
        [repeat for each field]
        </dl>
    </div>

In CRM/Admin/Form/Setting/Payment.php:
- around line 59, specify what fields should be shown for the payment
  processor.
- Add: showHideByValue('paymentProcessor', processor(s), processor div id, 'block', 'select', false);
  - this will not work if a showHideByValue() is added after a statement which
    starts with "return"
- Also add the fields added to the template below this line.

- showHideByValue (
    trigger_field_id
    trigger_value
    target_element_id
    target_element_type
    field_type
    invert
  )
Begin your file in CRM/Core/Payment.

The params available to doDirectPayment() are:
- qfKey
- email-(bltID)
- billing_first_name (=first_name)
- billing_middle_name (=middle_name)
- billing_last_name (=last_name)
- location_name-(bltID) = billing_first_name + billing_middle_name + billing_last_name 
- streeet_address-(bltID)
- city-(bltID)
- state_province_id-(bltID) (int)
- state_province-(bltID) (XX)
- postal_code-(bltID)
- country_id-(bltID) (int)
- country-(bltID) (XX)
- credit_card_number
- cvv2
- credit_card_exp_date
  - M
  - Y
- credit_card_type
- amount
- amount_other
- year (credit_card_exp_date => Y)
- month (credit_card_exp_date => M)
- ip_address
- amount_level
- currencyID (XXX)
- payment_action
- invoiceID (hex number.  hash?)

bltID = Billing Location Type ID.  This is not actually seen by the payment
class.

Also needed is:
CRM/Event/Payment/
CRM/Contribute/Payment/

important functions:
__construct - should be obvious
doDirectPayment - payment method in which CC info is entered on the web page
checkConfig - defined as abstract in the parent class, find config errors

doTransferCheckout - for redirecting user to a remote site to enter CC info.
    This must be done in each individual module, not in Core, to allow
    distinct URL's for the response page.

IPN = Instant Payment Notification

CiviCRM License: http://www.rosenlaw.com/AFL3.0.htm