How do I highlight a circle in the ground?

I want to make a mortar weapon that fires shots directly at an area in the ground. I need a visual indicator of the area being targeted that follows the terrain elevation, how should I do that?

Decals. Just take the location your mortar is targeting, then place an actor with a decal facing down at that spot.

This sounds like what I’m looking for. Could you share the details about how this goal could be achieved? I trying to find the tutorial or text description, but there is no anything I’m looking for.

Just to clarify: I trying to find how to display such a white circle (spell’s cast zone) like in the first two GIFs here:
https://www.artstation.com/artwork/VdB8PR