From Cuzco, take Road #3 east past the village of Sayalla (10-12 miles), then follow signs to the archaelogical site of Tipon (3500 m).
Park at the site, the proceed past the gate on the Inca Trail. The mountain comes into view in roughly three hours give or take depending on which route you take.
Bushwacking may be required below treeline depending on the route you take. Once past the Inca trail, there are multiple trails but most are overgrown. As long as your general direction is north to northwest and up, you are headed in the right direction. Be prepared for very thorny bushes. Around the treeline you will pass an impressive pre-Inca site. Head west past the site, then north again, straight up and past several farming areas. We went directly over the potato fields. You will eventually see the triple summit of Pachatusan looming above. Head straight up the grassy slopes. Depending on your route, you may have to walk down before heading up the slopes. Take your pick as far as which of the 3 summits you would like. I don't know what the exact altitudes are for each. Views from the top include Sacred Valley and Ausangate (6400 m). You can retrace your steps on the way down, but we took a different (and easier route) which does avoids most of the bushwacking and stays on the established trail longer. This route passes directly through the village, then cuts across fields until it intersects the Inca trail.
Our round trip time from our vehicle was about 8.5 hours.
Plenty of water. Trekking poles may be useful. Machete would have been nice for the bushwhacking. Rain gear may not be necessary in the dry season, but I carried mine any way.
You may want to bring some extra food for the local kids. I felt bad that I did not have anything to give them. Don't give them money or candy - the first creates bad habits, second ruins their teeth.
If you have information about this route that doesn't pertain to any of the other sections, please add it here.