There isn't always time or opportunity for lengthy hand-sewn repairs. In such cases, iron-on patches can be perfect for a quick fix for a hole or a stain.
These are sold in packs of 3, in the colors shown.
Although they are designed to be ironed on as they are, I recommend also sewing them on if you have the opportunity, as it will be much more durable. However, you can always iron them on quickly first if needed, and then sew them on at a later time.
It's easiest to do this on a machine since the patches are stiff, and it might be worthwhile to use a darning foot if you have one, and then disengage 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 there's a very large worn area with a small hole, I would recommend first attaching a reinforcing patch to the inside of the garment, either with machine stitching (a bit like quilting with stripes or squares, perhaps with thread that matches the garment?) or by using a fabric scrap and fusible web, ironing that on the inside, and then covering the hole or stain with the flower. Of course, you can also use iron-on patches to decorate clothes that don't yet have any damage or wear :)
Reinforcing worn areas from the inside 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 stiff material against the thin/worn fabric.