From Guatemala City drive south via CA9 and then take the CA2. The road has signs at every turn you could find but keep your eyes open. Very often people end up on the road to the beach.
You will reach the entrance to the main city of Rehu, stay on the main road (CA2) and keep going. After 15 minutes approximately you will reach a turn to your right taking you to El Asintal. Take this turn and you are