Create a new module, say 'sale_customization' in the addons directory and create __init__.py and __openerp__.py files in the module. Then create a file sale.py in it and add the following code:

from openerp.osv import fields,osv

    class sale_order(osv.osv):
        _inherit = 'sale.order'
        _columns = {
              'field1': fields.char('Field1', size=64), 
              'make_of_car': fields.selection([('make1', 'Make1'), ('make2', 'Make2')] ,'Make of the Car', required=True),
              'model_of_car': fields.selection([('model1', 'Model1'), ('model2', 'Model2')] ,'Model of the Car', required=True),
       }
     sale_order()

Now add the line import sale in __init__.py file. Then create a file sale_view.xml in the module and add the following in it:

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record id="view_order_form_inherit" model="ir.ui.view">
            <field name="name">sale.order.form.1</field>
            <field name="model">sale.order</field>
            <field name="type">form</field>
            <field name="inherit_id" ref="sale.view_order_form"/>
            <field name="arch" type="xml">
                <data>
                     <xpath expr="//field[@name='origin']" position="after">
                        <field name="field1"/>
                        <field name="make_of_car"/>
                        <field name="model_of_car"/>
                    </xpath>
              </data>
          </field>
  </data>
</openerp>

Now add the following to the __openerp__.py file.

{
    "name" : "Sale Customization",
    "version" : "1.0",
    "author" : "Gopakumar N G",
    "website" : "http://www.zbeanztech.com",
    "category" : "Custom",
    "depends" : ["sale"],
    "description": """ Sale customizations """

    'init_xml': [],
    'update_xml': [
        'sale_view.xml',
    ],
    'demo_xml': [
    ],
    'test': [
    ],
    'installable': True,
    'active': False,
}

Now restart the server and update the module list in Administration->Modules->Update Module List , the in Installed Modules menu uncheck the 'Installed' filter and select your module and install it.