The cheapest and best way is to organize the official tour from the planetarium. Call there and you can book your trip. It starts from Cuzco Place regocijo: Website of the planetarium;

The transportation is organised by them, they pick you up in a square near the main square of Cusco and bring you back to the same place.

Transportation was provided as part of our package

As the others said, the hotel can arrange transport through various travel providers. Don't miss it, its spectacular!

All of our transportation was arranged by our tour company, Enigma Travel. I highly recommend them.

This is a family owned and operated endeavor. My Spanish language school arranged my tour. The actual astronomer himself and his nephew came out to pick up the group of tourists who had signed up for tours. I'd look for a tour and they'll set up transportation for you from the city.

Bring cash, and bring change. They had some trouble making change when I was paying.