it's easy making a lightning, first, make a 'info_target' and call it "lightning1", this is the place where the lightning starts
then make another 'info_target' and call it: lightning2
that's the place where the lightning ends
then make a 'entity_beam' and fill in the following options:
Start entity / Ending entity: the names of the 'info_targets' so: lightning1 and lightning2
Brightness: something between 1 and 255, i take 128
Beamcolor: the color of the beam ofcourse
Life: fill in '0'
Width of beam: fill in '128'
Amount of noise: fill in '140'
Sprite name: fill in "sprites/lgtning.spr"
Frames per 10 seconds: fill in '0'
Strike again time: the time in seconds that the lightning appears again...
Damage / second: the damage it does per second standing in it..
I hope u understand this... good luck!