Decorating pottery can be done using the sgraffito technique of incising down through slip. This begins by applying a coating of a contrasting colored slip to the damp or leather-hard clay. The slip and clay are then allowed to stiffen until both are leather-hard.
At that point, designs or patterns are incised or carved down through the the slip into the clay body beneath. The designs are enhanced due to the contrast between the color and tonal qualities of the slip and clay.
The character of the lines and carving will be effected by the tool(s) used and the dryness of the slip and clay when the incising is done. Experimentation is the key to finding what owrks best for the effect you want.