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.
Although they are designed to be ironed on as they are, I recommend also sewing them on if possible, as this 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 to do this with a machine since the patches are stiff, and it might be helpful 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 need 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 squares, perhaps with 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 with the stiff material against the thin/worn fabric.