The quickest way to do it is:
1. Open the master form in Design view. At this stage it shouldn't have the child form on it -- delete if it is. Restore so you can see other items on the screen.
2. Press [F11] to bring the database window to the front. Navigate to Forms, find the subform you want to use, click and drag onto the main form, and release. You'll know it worked correctly if the main form highlighted in reverse video before you released the subform.
All being well, the forms should synschronise. If not, back to design view. Click the subform ONCE, click the Properties button on the toolbar, and you should see the Link Child Fields / Master Fields properties. If they are not populated, click in either one, hit the Builder (...) button and select the link fields in the dialog.
Denis