It's not always possible to spend time on lengthy 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 are designed to be ironed on as is, I recommend sewing them on as well if you have the opportunity, as it makes them much more durable. However, you can iron them on quickly first if needed, and then sew them on at a later time.
It's easiest to do on a machine since 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 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 reinforcing patch to the inside of the garment first, either with machine stitching (a bit of a quilt style with stripes or checks, perhaps with thread that matches 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 have no damage or wear!
Reinforcement on the inside of worn areas is beneficial so that you don't get a new hole exactly at the edge of the iron-on patch, where there will be extra stress from the stiff material against the thin/worn fabric.