the easiest way is to create your form, then create your subform.
Then in the design of your main form, use the subform control ( on the control toolbar) it will act as a wizard. select your existing subform, and then it should show you what it can see as fields to link on. If you don't like them, go to the subform properties, as it sits in the main form design view. In it properties, set the master field and child field links as you wish.
misscrf
It is never too late to become what you could have been ~ George Eliot