Heat the fabric before placing the patch on it, covering it with a cotton cloth. Press down firmly on the patch with your iron until the glue melts (allow at least 15 seconds).
Wait a moment for it to cool down, then check by gently lifting one edge of the patch with your fingernail to test the adhesion. If necessary, repeat the process, pressing down firmly with the hot iron.
Turn your garment over and press down firmly again on the back of the fabric.