Step-1} First we have to open a NEW unity project Create project UNITY 3D.

Step-2} Next step is to import the UNITY assets there are unity assts which should be imported

              Here are names and links to import them.

• Environment

https://assetstore.unity.com/packages/3d/environments/sci-fi/free-lowpoly-scifi-...

• Shooting Gun

https://assetstore.unity.com/packages/3d/props/weapons/low-poly-fps-pack-free-sa...

• Enemy 

https://assetstore.unity.com/packages/3d/characters/humanoids/fantasy/skeleton-o...

Step-3} Next step is to place all of them to create our shooter game by placing the environment in the  scene and adjust the lights in the environment.

Step-4} Now place the Gun and the enemy in the game scene.

Step-5} Now, enjoy the game.