It looks like an ordinary throw, except the design is printed as outlines that you colour in yourself. The usual size is close to 150 x 125 cm, and the pens are often included.
You pick a section, colour it in and come back to it another evening. 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. Choosing the design is half the fun, so it is worth looking at the whole pattern first.
The question people ask most is about washing. With water-based washable pens, manufacturers state that the colour comes out in a normal machine wash, so the same design can be coloured again. Check the care label before the first wash.
Some people switch to permanent pens for a finished blanket they want to keep as it is. Just keep track of which areas were done in which ink.
A few practical tips help. Lay the blanket flat on a table rather than on your knees, and put the caps back on the pens between sessions. Testing a colour on a small corner shows how it looks on the fabric.
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.