Making An Env_Beam

First of course build your map. Then place an env_beam entity in your map, most likely next to where
the beam will emit from
so you can find it later on if you have to edit it.
Then you can make the brush where the beam will start from and make it a func_brush(any entity will do)
entity and name it. Then place an info_target entity (again any entity will do) at the spot where beam
will end it's travel and name it.

Ok let's start with the env_beam entity Properties
There are many options, you can select the color of the beam, parent it to another entity, etc.
Main ones are width of beam, amount of noise (more noise = more erratic of a beam(lightning),
less noise = a straight line), you can browse thru the different sprites available for it also.
Then you need to select the Start Entity and then the Finish Entity. Which is where the beam will
start and finish at. You can also name the env_beam and have something trigger it on or select random
strike in flag options.

Also look at the flags tab and check off options wanted such as Start On

Then simply name your Entity where beam will start (func_brush in my example)

Then name your finish entity (info_target in my example)

In Hammer Editor the lines should connect showing you the entity path. (which entitys are connected).
Save map, compile and test it out, then adjust to your liking using the many options in the property

you can add env_sprite to give it a glowing appearance (seen in picture above).