It's not always possible to do time-consuming hand-sewn repairs. In such cases, iron-on patches can be perfect for a quick fix for a hole or a stain.
Even though they can be ironed on as they are, I recommend also sewing them if possible, 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 time.
It's easiest on a machine since the iron-on patches are stiff, and it can be beneficial to use a darning foot if you have one, and then disengage the feed dogs (upper and lower feed) so you can move the fabric/garment in any direction you want. Feel free to practice on a fabric scrap 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 reinforcement patch to the inside of the garment first, either with machine stitching (a bit like a quilting style with stripes or checks, perhaps with thread matching the garment?) or by using a fabric scrap and fusible web (vliesofix), ironing it 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!
Reinforcing the inside of worn areas is beneficial so you don't get a new hole right at the edge of the iron-on patch, where there will be extra stress due to the hard material against the thin/worn fabric.