Think of a colouring book, but made of fabric and big enough to cover your lap on the sofa. 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 one of the few hobbies a whole household can share on the same piece.
White lines on black make every colour stand out, even the lighter ones. Large areas fill quickly; small detailed zones take longer but look striking when finished.
The key detail is the ink. Washable pens use water-based ink that, per the manufacturer, washes out in the machine, so you can start again. The care label sets the temperature and drying.
Some people switch to permanent pens for a finished blanket they want to keep as it is. You can mix both on the same project, as long as you remember which parts will stay.
A few practical tips help. 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.
It is an easy way to keep your hands busy while chatting or listening to music. It is also a popular Christmas gift idea, as it is still useful once the colouring is done.