Take the full hike starting from Llupa and then keep going to you meet the intersection in Pitek. You'll take a combi from Huaraz for the price of 3 soles. When you get further up to the rocks there are 2 paths, the one on the left has cables to climb up the rocks along the waterfall. The path on the right doesn't have cables but is slightly easier on foot.
Lake Churup is a great hike to acclimate before doing the Santa Cruz Trek. Lake Churup sits at 4450m/14,600ft.