From Marrakech cut off on the R210 after about 10km, it's an easy quiet road heading for Demnate. When you get to Demnate go south a few km to visit Imi n'Ifri and spend 40 minutes or so walking down and through. Yes, you could stay in Ouzoud, alternatively a few miles after Azilal on the edge of the reservoir Bin el Ouidane there's a couple of waterfront hotels.
At Khenifra turn right (east) for 25km. You gradually ascend into a cedar forest. Turn left (north) signposted Aguelmane Azigza and Source Oum er-Rbia. The Oum er-Rbia is Morocco's longest river and emerges from the rockface fully formed in a series of powerful springs, so park up and spend an hour or so walking uphill to the springs and exploring. See photos at http://looklex.com/morocco/oum_er-rbia.htm
From there keep heading north following signs for Ain Leuh. It's a lovely drive through the cedar forests and limestone karst scenery. Keep an eye open for signs of small furnaces by the side of the road where the lime is converted to quicklime used in building mortar.