I’m not gonna bother making a guide, because plenty of people have made very fine ones for us already.
If you haven’t decided which one you’re going to use yet, I’d suggest trying the second one (Mik’s) because it’s been updated most recently and is just one big addon, as opposed to several smaller ones linked together. At least, that’s the reason I use that one. Lots of people like SCT, though, and I believe it’s been around longer.
Once you’ve done this walk-through for L&L, it’ll be easy enough to plug in other events or spells that you want similar warnings for.