how to make SSAO?

From depth channel?

I found SSAO node but whatever I do with it it always turns red.

Could somebody give me a tutorial link with some explanation.

The SSAO node seems to require depth and normals channels along with the input image and a camera input. Then it should work.
For example, if you're rendering out an image from a Fusion3D scene with the Render3D(3Rn) node, make sure to enable Normal and Z, and then pipe in your scene camera.
Hope that helps!

Thank you very much for the hint. It works as you suggested and renders very quickly. For some weird reason I can't force it work with exr channels