Starting from the Las Grutas border control post.
If you're lucky you can catch a ride with some archeologists to base camp and in that case it takes about 40-50 minutes. Walking is 3 hours for a fast walker. Be sure to bring plenty of water to the base camp as there is none there.
If walking, just follow the jeep track to a place where you see many tent platforms and other traces of "camp life". If climbing in the main season this place is for sure full of scientists. The base camp is at just over 4300m.
Continue along the jeep path until it disappear. It does when it's too steep for driving at about 4600-4700 depending on which of the two main tracks you follow. There are two main ridges in front of you. Aim for the one in the horizon to your right, not the one in front of you. There is a huge couloir in between and this is where you will find snow and ice to melt. Snow begins at 5000-5050m and there are some good camp spots at 5300m. This is a good place for camp 1. The couloir is very hard to walk because of the white volcanic gravel.
The elevation where I found snow was perhaps during a snowy year/season. Another climber found no snow to melt below 5500m.
You better aim straight for the north ridge. Another reason to do this is to avoid difficulties to access the ridge further up. Only a few places let you get up or down without technical gear. Walk along the ridge, straight up until you hit a multicolored rock band. Walk over to the left side and stay very close to drop to the couloir. It is much easier rock here and if you go right, you have to go quite technical in parts. When you have passed this section it's not long to the plateau. It flattens out and for the first time you will see the summit of Incahuasi. No problem here, just go along the path of least resistence. The summit's south side is very steep and some rocks are loose. Be careful!
No technical gear needed. Walking poles are almost a must on the loose gravel. Crampons can come in handy on some icy and snowy parts. The walk to the peak is in wide valleys. It's easy to loose the track. Bring a GPS. Bring lots of water from Las Grutas!
Warm clothing and good sun glasses is a must.