The short answer
Ras, steamed sheep's head, is sold at a few specialist stalls in Jemaa el-Fnaa in the evening. The head is steamed until the cheek meat falls away, then served with cumin and salt and eaten with bread. Ask for cheek to start, the easiest cut, and point; the cook does the carving.
In more detail
The whole head is steamed for a long time, until the meat is soft enough to pull away with bread. The cook lifts it out, carves to order and dusts it with cumin and salt. That pinch of cumin is the only seasoning it needs.
The cheek is the prize and the easiest place to start: soft, rich and closer to slow-cooked lamb than anything unusual. Tongue and brain are there for the curious. You eat it with your fingers and a piece of bread, sitting at the stall's bench.
It is an evening and late-night dish, sold as the night market runs from about 16:00 until about midnight. As with every stall in the square, agree what you are ordering and the price before you sit.
If you are curious but nervous, it is easier with someone who knows the stall. A sheep's-head specialist is one of the classic stops our evening food tour can include, and your guide will order the right cut for a first try.