How to do to reduce static mesh tris after creating an imposter material?

People told me that I can reduce static mesh tris by using imposter.

Then I follow the document: Render 3D Imposter Sprites | Unreal Engine Documentation
I create an imposter material, then what to do next?

And there is my display. I don’t what to do. So I use the material I created to set my static mesh’s material.