What did you try already? In general you should be able to add an Insert Server Behavior to your checkout page. In the server behavior you should be able to select the table to insert into, then select the columns and the corresponding form field that you would like to store in that column.
If you use the store order summary server behavior you can get mixed results in regards to updating an existing record. Please post back with what you have tried and which WebAssist tools you have access to.