Lay your cloth on a flat, heat-resistant surface like an ironing board. To ensure the item will provide a good surface for the patch, iron it first. If it's a backpack or another item that's difficult to iron, do your best to arrange it so that the part of the fabric that will be receiving the patch is flat against a hard surface.
Place the patch in the position you chose. The adhesive side should be flat against the base fabric. Make sure that the patch isn't crooked. On embroidered patches, the adhesive side is the underside.
Heat up an iron. Turn it to the hottest setting your fabric can tolerate. Make sure the "steam" option is turned off, and that your iron isn't full of water.
Place a thin towel over the patch. Take care not to disturb the position of the patch. The towel will protect the patch itself and the surrounding fabric.
Position the heated iron over the patch and press down. Hold the iron there for about 15 seconds. Apply as much pressure as you can by pressing down firmly.
Remove the iron and allow the patch to to cool. Lift the towel and check to see whether the patch is fixed on securely by gently rubbing at the edge with a finger, trying to lift it. If it lifts up a bit, replace the towel and press it with the iron again for 10 seconds