Magento Onestep checkout

I would like to add a custom field under the billing information tab of magento onestep c开发者_如何学运维heckout. Does any one know how i could get this done. I am pretty much new to magento

Thank you

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Really depends on what you want to do with this custom field: Is this part of a payment method? Then extend that payment method, add the field to the template for said module and later on save the attribute in the model.