you guys are way overthinking this.
Obviously he had at least some gun knowledge but saying he needed to have had some kind of training to pull this off is a bit of a stretch.
He could've pretty much planned this whole thing in literally 10 minutes.
How's this for a scenario? He's on one of his previous trips to Vegas, (we know he was an avid gambler who practically lived in the casinos,) up there in one of these rooms overlooking the fairgrounds, maybe even in the very same room. maybe there's even an event going on at the time, and he suddenly decides, "you know what, I'm gonna shoot the crap out of this place one day."
ok google, what's the effective range of an AR15? I'm good on that front, check.
What upcoming events are there at Las Vegas village? Oh there's a country music festival? check.
How can I convert an AR15 to full auto? Oh I can get a bump stock legally? check. (he probably knew/had these already)
And there you go, he's pretty much done. Of course there's the small logistical details to work out, but the general idea is here.
This doesn't mean he didn't have any training etc, I'm just saying it wasn't necessary.
ETA: Edited to clarify that the hypothetical scenario presented occurred on a previous visit, not right then.