Divide the grid into areas and write a number in every field. The numbers in the same area have to be the same and have to tell the number of fields in that area. Areas of same size my not touch horizontally or vertically, but diagonally. Given numbers may belong to the same area, and it's possible that there are areas, where no number is given, even with larger numbers as the once shown.
The clues along the edges tell you how many skyscrapers you can see from that vantage point.