It's not always possible to spend a lot of time on extensive hand-sewing repairs. In such cases, iron-on patches can be perfect for a quick fix for a hole or a stain.
Even though they are designed to be ironed on as is, 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 later.
It's easiest to do this with a machine since iron-on patches are stiff. 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 scrap 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 recommend first attaching a reinforcing patch to the inside of the garment, either with machine stitching (perhaps a quilted style with stripes or checks and 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!
Reinforcing worn areas on the inside 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 from the stiff material against the thin/worn fabric.