Leave your vehicle at the parking, and then ascend the hill following the fence up to the trailhead. From there the path is clearly visible all the way up to the summit.
The distance from the parking to the summit is almost 2 km, and it will take about 40-60 minutes to walk up there. Expect that the complete tour takes 3 hours, since you probably want to stay a while at the summit.
Up on the summit you will on an clear day have an extraordinary view over the surroundings.
The guardian of Slættaratindur.
If you are lucky, you can on the way up get an glimpse of Slættaratindurs guardian. The guardian is an very shy creature and very few people has ever seen it.