The twins - Pomerape and Parinacota.
First: The mountain is over 6300m and therefore the altitude is a consideration.
In Northern Chile the nearest really large city is the capital of the Arica Province - Arica, which is located at the Pacific coast. Leaving Arica, you head for Putre 125km up Chilean Highway # 11. You're now at 3600 meters above sea level and it's a good idea to have a break here for a day or two.
Continue for another 44 km to the little village Parinacota (4400m) and continue to Caquena where you have to register your intention to climb at the police post. Go back along the road you came and turn off at km 26 from Parinacota Village. The road is sandy and paritally in a very bad condition. Follow the obvious track around the swamps until you arrive in a place called Rinconada. A shephard lives here and it's a good place for fresh water. The water in the nearby swamps should only be drank in an emergency situation.
The long walk through sandy areas takes you to the foot of the Volcano's western and later northern approaches. Even though the track can be hard to find, you'll have a hard time to loose a 6000m plus high volcano. Make sure to reach a snow field so you can melt some for water. Very hard to find any running water here. There are quite a few possible camp spots with good shelter close to the snow fields. During the climbing season, 4700m - 5100m is the normal snow line .
There are many high points
Start the morning of the summit attempt with having a close look at where you want to climb. The mountain is very symmetrical, but there are some areas steeper than others and it's easy to end up in places which are a bit too steep for normal scrambling. Another thing to consider is to avoid the areas with too hard ice. There are paths with very hard blue ice which are hard to cross.
Start up the NW "face" and move towards the E and and when there's a quite obvious flatter area, head for the highest point. The summit can't be seen from here.
When you have passed the steeper last parts, you're on the crater rim and from here you have to follow it to the right. The summit is easy to distinguish from the lower points on the rim, even though there are other points which are almost as high. See the above photo for all the small summits.
Note: the official elevation of Parinacota is 6342m and differs from the elevation given in the photo.
My guess is the average of the route is about 35 degrees and possibly up to 45 degrees at the steepest point. That is, if you manage to stay away from some steeper areas on the way up.
I have heard reports about crevasses, but have a hard time to believe that statement. Take care anyway, I may be wrong. If there are any, they must be quite high up on the peak, close to rim.
Essential GearWater and wind proof clothing.
Warm boots, which you can put crampons on.
An ice axe.