I'm looking for the file name containing the implementation of the "Proceed to Checkout" button.

I have found that the `sales_flat_quote_item` table is being populated when the button is clicked. I have added a field called `fld_data` and now I need to insert the field data, currently held in a session, when the "Proceed to Checkout" button is clicked, in the same insert statement.

---

Sorry for the misunderstanding actually i want to add a custom option textfield which is hidden from the user and process it till the order submitted and that value is accessable at the admin side along with the order.