Just add one field in your table to accept a number byte data type will work.
Here is where you will store a value from 1-12.
To populate or change this value in a customer form, just enter the value directly or use a combo box containing the values 1-12.
If you really want the check boxes, then place them in an option group with each unbound check box have the value it need to populate this new field.
Then on the after...