How to make a bas relief?

To add more info to the first answer (which is very informative), But I'll add: take the slab of clay, then draw onto it with a sharp tool. Then add a thin slab of clay inside of the drawing so that your drawing of Ganesha now "pops" out from the flat slab. Be careful to not have any undercuts (no overhanging forms that would catch your finger). Then, look again at the photo or drawing that you are recreating. And decide what are forms that come forward and what are forms that are layered over each other. Do the arms overlap the torso? Do the knees come forward? Is the belly round? If the arms come in front of the belly, then add another thin slab of clay to create the arms. Continue to add layers to make your figure "pop" out in 3D from the slab...but make sure that all these forms are thin slabs and that you are compressing ALL the forms...this is what makes it a bas relief and NOT a normal sculpture!