Mandalas generally begin with the drawing of a center. The monks (or other artists) make small lines at the top and bottom, then more lines at the other poles, then more lines until they have eight sections. Then the monks can add anything. A shape, a splotch of color, a letter, anything that comes to mind. The point is to repeat this eight times, during which time the monks meditate on the shape or color that emerges in their minds. You can create your own mandala with pencils, pen, crayon, chalk, paint or other mediums.
Some people quilt mandalas. They begin with the center, then cut a shape out of cloth to add to the quilt. As a mandala grows, it may not always be eight of the same shape. But it is always a pattern within a circle. As you make a quilt, the pattern should grow from the center and fit within a circle.
Tibetan monks make sand mandalas with colored sand. The monks sometimes travel to different cultures, making traditional mandalas with symbols that must be replicated perfectly. They take days to make. When the monks finish, they sweep the sand into a jar and pour it over a body of water to bless it. The whole ritual serve as a meditation on the transient nature of life.
It is possible to download mandalas from the Internet that others have made and to color them or fill them in with personal symbols. This might be a good way to introduce someone who has difficulty with making art to the concept and process of a mandala.