Hello Readers,
In this tutorial, i am going to explain How to add a new Custom Shipping Method to existing Magento Installation which will accept value (i.e shipping charges) from System->configuration option. Here i have used Custom as a module name. You can create your Own module name & change class names accordingly.

Lets start by creating the following folders:
app/code/local/Custom/Customshippingmethod/etc
app/code/local/Custom/Customshippingmethod/controllersr
app/code/local/Custom/Customshippingmethod/Model

1.  create the system.xml (under etc folder) file for admin entries.

<?xml version="1.0" encoding="UTF-8"?>
<config>
  <sections>
    <carriers>
      <groups>
        <custom_customshippingmethod translate="label">
          <label>Custom Demo Shipping Method</label>
          <sort_order>1</sort_order>
          <show_in_default>1</show_in_default>
          <show_in_website>1</show_in_website>
    
            <show_in_store>1</show_in_store>
    
          <fields>
            <active translate="label">
              <label>Enabled</label>
              <frontend_type>select</frontend_type>
              <source_model>adminhtml/system_config_source_yesno</source_model>
              <sort_order>1</sort_order>
              <show_in_default>1</show_in_default>
              <show_in_website>0</show_in_website>
              <show_in_store>1</show_in_store>
            
      
            </active>
            <title translate="label">
              <label>Shipping Method Name</label>
              <frontend_type>text</frontend_type>
              <sort_order>20</sort_order>
              <show_in_default>1</show_in_default>
              <show_in_website>0</show_in_website>
              <show_in_store>0</show_in_store>
              <show_in_store>1</show_in_store>
    
            </title>
            <sallowspecific translate="label">
              <label>For selected countries only</label>
              <frontend_type>select</frontend_type>
              <frontend_class>shipping-applicable-country</frontend_class>
              <source_model>adminhtml/system_config_source_shipping_allspecificcountries</source_model>
              <sort_order>30</sort_order>
              <show_in_default>1</show_in_default>
              <show_in_website>0</show_in_website>
              <show_in_store>0</show_in_store>
              <show_in_store>1</show_in_store>
              <show_in_store>2</show_in_store>
            </sallowspecific>
            <specificcountry translate="label">
              <label>Ship to Specific Countries</label>
              <frontend_type>multiselect</frontend_type>
              <sort_order>31</sort_order>
              <source_model>adminhtml/system_config_source_country</source_model>
              <show_in_default>1</show_in_default>
              <show_in_website>0</show_in_website>
              <show_in_store>0</show_in_store>
              <show_in_store>1</show_in_store>
              <show_in_store>2</show_in_store>
              <can_be_empty>1</can_be_empty>
            </specificcountry>
            <price translate="label">
              <label>Price</label>
              <frontend_type>text</frontend_type>
              <sort_order>32</sort_order>

              <show_in_default>1</show_in_default>
              <show_in_website>0</show_in_website>
              <show_in_store>0</show_in_store>
              <show_in_store>1</show_in_store>

            </price>
          </fields>
        </custom_customshippingmethod>
      </groups>
    </carriers>
  </sections>
</config>

Here active, title, sallowspecific and specificcountry tags are automatically handled by Magento

2. create a file config.xml (under etc folder) of custom module i.e (Customshippingmethod) and add the below code:

<?xml version="1.0"?>
<config>
  <modules>
    <Custom_Customshippingmethod>
      <version>1.0</version>
    </Custom_Customshippingmethod>
  </modules>
  <global>
    <models>
      <custom_customshippingmethod>
        <class>custom_Customshippingmethod_Model</class>
      </custom_customshippingmethod>
    </models>
  </global>
  <default>
    <carriers>
      <custom_customshippingmethod>
        <active>1</active>
        <sallowspecific>1</sallowspecific>
        <model>custom_customshippingmethod/demo</model>
        <name>Demo Shipping Method One</name>
        <price>10.00</price>
        <title>Demo Shipping Method</title>
      </custom_customshippingmethod>
    </carriers>
  </default>
</config>

<default> tag is used to assign default values to our system.xml fields( Here we specify default values for active,title,name,price etc.)
<model> tag contains the path of our shipping model.

3. Create shipping model class. The model class name is Demo.php and is created inside Model folder.

<?php

class Custom_Customshippingmethod_Model_Demo
extends Mage_Shipping_Model_Carrier_Abstract
implements Mage_Shipping_Model_Carrier_Interface
{
  protected $_code = 'custom_customshippingmethod';
 
  public function collectRates(Mage_Shipping_Model_Rate_Request $request)
  {
    $result = Mage::getModel('shipping/rate_result');
    $result->append($this->_getDefaultRate());
 
    return $result;
  }
 
  public function getAllowedMethods()
  {
    return array(
      'custom_customshippingmethod' => $this->getConfigData('name'),
    );
  }
 
  protected function _getDefaultRate()
  {
    $rate = Mage::getModel('shipping/rate_result_method');
     
    $rate->setCarrier($this->_code);
    $rate->setCarrierTitle($this->getConfigData('title'));
    $rate->setMethod($this->_code);
    $rate->setMethodTitle($this->getConfigData('name'));
    $rate->setPrice($this->getConfigData('price'));
    $rate->setCost(0);
     
    return $rate;
  }
}

This module has been tested for community edition 1.9.2.4  and you can download module zip file here.