From Latacunga take a bus to Chugchilan village (3200m) via Sigchos (3900m) at 10 am, or via Zumbahua (2920m) at noon. The latest is a tough road.
From Chugchilan (3200m) decent into the canyon of Rio Sihui which has to be forded. Then hike up on the other side on the trail which is a deeply eroded notch. On the other side of the canyon you will see the village of Huayama. Cross the village from north to south.
Once out of Huayama target the big sandy spot you see at the crater rim (look south). You will reach the crater on its northwest side. The summit is at the opposite side of the rim and you have the choice to go for it clock or counter-clock wise (15 km, + 1000m from Chugchilan).
Wind proof and warm clothes. It can get quite windy on the crater.
Watch for dogs. They may chase you. In this case do not run. Best thing is to simulate throwing them a stone, or actually doing it. However, no need of a one kilo stone, a small one should suffice!