I recommend you begin at 12,100 feet on the Avenue of the Volcanoes Highway a half a mile before West entrance to Cotopaxi National Park where buses sometimes park with guided climbers on the turnouts. Aim towards a public bridge to cross the river - don't cross on private land and be aware if you get off route you'll be on private property. None of the approach and climb is in the National Park. If you enter the National Park you're required to have a guide even if climbing Sincholagua which is outside the park. There's a shorter alternative approach from the North (mentioned in several guidebooks) that people used to use but I was told by locals that it's on private property and the farmer that owns it will literally shoot all trespassers including climbers. I highly discourage you from taking that North route both for your own safety and out of respect for the wish of the landowner. Be sure to get an early start of hiking the approach and the climb in one day because thunderstorms & fog are very common in the afternoon as in almost every day! There's good options for overnight camping.