Sometimes you don't have the time or opportunity for time-consuming hand-sewing repairs. In that case, 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 if you have the option, as it makes them much more durable. However, you can quickly iron them on first if needed, and then sew them on at a later date.
It's easiest to do on a machine since the iron-on patches are stiff, and it might be beneficial to use a darning presser 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 there's a very large worn area with a small hole, I would recommend attaching a reinforcement patch on the inside of the garment first, either with machine stitching (perhaps a bit of a quilt style with stripes or checks using thread that matches the garment?) or by using a fabric patch and Vliesofix, 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 have no damage or wear!
Reinforcement on the inside of worn areas is beneficial so that you don't get a new hole right at the edge of the iron-on patch, where there will be extra stress from the hard patch against the thin/worn fabric.