One doesn't always have the time or opportunity for time-consuming hand-stitched repairs. In such cases, iron-on patches can be perfect for a quick fix for a hole or a stain.
Although they are designed to be ironed on as they are, I recommend also sewing them on if possible, as this will make them much more durable. However, you can always iron them on quickly first if needed, and then sew them on at a later time.
It's easiest to do this with a machine since the iron-on patches are stiff, and it might be beneficial to use a darning foot if you have one, and then remove the feed dogs, so you can move the fabric/garment in any direction you want. Feel free to practice on a piece of fabric with a ballpoint pen drawing of a heart or star or something. The stitches don't have to be perfectly even; the most important thing is that the patch is securely attached :)
Iron-on patches are a bit stiff, so if it's a very large worn area with a small hole, I would recommend attaching a reinforcing patch to the inside of the garment first, either with machine stitching (a bit like quilt style with stripes or checks, perhaps with thread matching the garment?) or by using a fabric patch and Vliesofix, and ironing it on the inside, and then covering the hole or stain with the iron-on patch. Of course, you can also use iron-on patches to decorate clothes that don't have any damage or wear and tear!
Reinforcement on the inside of worn areas is worthwhile so that you don't get a new hole right at the edge of the iron-on patch, where there will be extra stress with the hard material against the thin/worn material.