1. Save the downloaded file on your pc, where you can find it later.
2. Open the blender file in which you would like to import the staircase system.
3. File --> Append --> ACS_Staircase Generator.blend --> NodeTree --> ACS - Spiral/Straight Stair Generator
4. Play with the input attributes in the modifier stack until you like how it looks.

The system is designed to work with auto smooth. So just right click the model and choose "Auto Smooth". It should look better now!

You have 2 options to make the edges of the stairs look more realistic: 

1. Use a bevel node in the material node tree and plug it into the normal input of your shader (Thats what I did in all of the preview animations on the store page)
2. Bevel the edges with a bevel modifier AFTER the geo nodes modifier (depending on edge angle)