In this video

Before installing the newly cut tread, a generous bead of construction adhesive is applied to the horizontal wood supports (ledgers) within the stair frame where the tread will rest. This adhesive (a caulk gun loaded with a tube of adhesive is shown) serves two primary purposes: to firmly bond the tread to the frame and to prevent future squeaks that can arise from wood-on-wood friction as the stairs are used. The adhesive is applied to all contact points, including the vertical stringer/riser faces.