Assuming the form already exists, open it in Design mode, and look at the
Properties for the form (View | Properties from the menu bar). Assuming
you've looking at the properties of the form itself (as opposed to a control
or section on the form), look at the Data tab. One of the properties is
"Record Source": set that to the name of the query you created.
If you're creating a new form, when you click on the New Form button, the
dialog that appears let you select the Record Source (there's a combo box
containing all of the tables and Select queries in the database)