You want to avoid your skyscrapers looking too regimented and similar - like this example!
To do this draw some basic skyscraper shapes but avoid making them all rectangles.
Then add additional shapes to them to make them look more interesting.
Once we have the basic shapes we can start adding windows and details. Try to avoid grids for your windows and instead use just horizontal or vertical lines.
Finally, add some smaller buildings at the front of your skyscrapers to create a sense of depth and scale.