It's not really a case of selecting nice-sounding entities that you've not used yet and sticking them in the map, y'know
Use env_renders if you need to in order to create an effect you want and prefer them to an alternative method for some reason, but you may find that you can do the same with fewer entities in another way
Globals are fairly tricky to get right, but once you've got the hang of them they're a great help and add another dimension to a game's mechanics
(But are often unnecessary). On de_slit they're used for the computer-blowing-up bit, if you've played it. Basically they remember the value of a setting and can return it when something asks, so triggering something will do one thing if the global setting is such and such, and another if it's something else.
Be careful with beams on your map, as they are laggy. (They definitely contributed to de_blockwars lag
) To create a random event generator, the point you need to know is that if they target a number of identically-named targets the beam will choose one at random. If you're so inclined you can be a bit silly and make a visible random generator, like this in Blockwar
I'll have a look today though!
Look for a texture types tutorial (on countermap maybe), or Wally even tells you when you paste a texture into a wad 