It looks like an ordinary throw, except the design is printed as outlines that you colour in yourself. Most measure around 150 x 125 cm and come with a set of pens in the box.
The design is large, so it works best a little at a time rather than in one sitting. It is wide enough for two or three people to work on it at the same time.
Many designs are printed as white outlines on a black background, which gives a stained-glass look once coloured. A busy pattern keeps you going for weeks, while a simple one is done in a few evenings.
The key detail is the ink. According to the manufacturer, washable water-based ink comes out in a normal wash, leaving the printed outlines ready for another round. The care label sets the temperature and drying.
Permanent fabric pens are for the reverse: a design you want to keep, which will not wash out. Just keep track of which areas were done in which ink.
Some simple habits make it easier. Colour on a flat surface so the fabric does not stretch, and close the pens after each session so they do not dry out. Start with lighter colours; darker ones are easier to add than to take away.
Many adults treat it as a relaxing hobby for evenings on the sofa. It is also a popular Christmas gift idea, as it is still useful once the colouring is done.