The best glues for attaching flat-back, non-adhesive rhinestones include Beacon Gem-Tac, Aleene's and other products that are specifically for use with rhinestones. Instant adhesives, glues that contain acetone, and hot glue guns are not appropriate for attaching rhinestones to cotton fabric.
Some rhinestones come with pre-applied adhesive and include application instructions from the manufacturer. These adhesives often require a hot iron to create the bond. There are many different kinds of rhinestones on the market and a variety of different techniques are effective to apply them to fabrics. Sewing the rhinestones onto the fabric or crimping them is more appropriate for some types.