With a tour operator
Of course, this is the simplest solution, but it may also be the most tiring, as you'll be getting up at around 3am, but this excursion is really worth it!
Here's an outline of a typical day:
Departure around 4am from your hotel in Cuzco by van or minibus to Mollepata.
Breakfast in Mollepata. Transport to Soraypampa.
A 3km uphill walk leads to Lake Humantay, approximately 1? hours.
Lunch at Mollepata before returning to Cuzco at the end of the day.
Price from €44, including breakfast and lunch and English or Spanish guide.