Think of a coloring book, but made of fabric and big enough to cover your lap on the couch. The usual size is close to 59 x 50 inches, and the markers are often included.
There is no rule about where to start: one flower tonight, a corner of the pattern tomorrow. Because the blanket is wide, two or three people can 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 colored. Large open areas go faster; small detailed zones take longer but look striking when finished.
What makes these blankets different is the washable ink. According to manufacturers, washable water-based ink comes out in a normal machine wash, leaving the printed outlines ready for a new round. Always follow the care label on your blanket.
Some people switch to permanent fabric markers for a finished blanket they want to keep as it is. Just keep track of which areas were done with which ink.
A few practical tips help. Color on a flat surface so the fabric does not stretch, and close the markers after each session so they do not dry out. Testing a color on a small corner first shows how it looks on the fabric.
For adults, it is a calm evening activity away from screens, in the same spirit as coloring books. It is also a common gift idea for the holidays, since it is useful long after the coloring is done.