Make sure that you remove all of the original surface of the tube where the patch will go; it contains a mold release compound which will interfere with the chemical reaction which fuses the rubber layers together. Don't just scuff the surface, expose all new rubber beyond where the edges of the patch will go. Keep your fingers off of this newly-exposed surface.

Coat the entire prepared surface evenly and fully with vulcanizing compound. Allow it time to dry completely without blowing on it. Make sure that the vulcanizing compound (NOT ordinary rubber cement) has time to thoroughly dry before pressing the patch into place. Don't rush this step.

After pressing the patch evenly and firmly onto the tube, "stitch" it down hard to make a firm bond, especially at the edges. Use something hard and smooth like the edge of a coin or the end of a tire lever and work it all along the surface of the patch pressing every bit of the surface against the tube. Any places you miss are potential leaks. After you think you are done go over the edges again.