1.Make a large map.
2.Place an entity named "env_mortar_controller" and name it mortar_controller.This will be the controller.
3.Place an entity named "env_mortar_launcher" somewhere outside the map and edit the properties to your like (and name it to mortar_launcher)
4.Place an entity named "point_camera" above the map and edit the options to the following:
Pitch Yaw Roll - 90 90 0
Screen _rt_ Camera Ratio - _rt_Camera
Name:mortar_camera
FOV - 60
Screen Aspect Ratio - No
Fog Enable - No
Fog Color - 0 0 0
Fog start - 2048
Fog End - 4096
Fog Max Density [0..1] - 1
(basicilly the options you get when placing it, but theres only a changed FOV 
)
5.Place an entity named info_camera_link and place in the Entity Whose Material Uses field _rt_camera your mortar name (in this case mortar_controller)
In camera name use mortar_camera.
6.Go to env_mortar_controller and fill in the Mortar Launcher field mortar_launcher.
Example .VMF can be downloaded here
Feel free to ask questions, since this is my first tutorial and its probably not really good to follow.
Have fun 
.
