I'm wondering how I should EV train an Umbreon with Curse. I was told Atk and HP...is that good, or should I work on something else?
Obviously, I'm very new to this and I'm second guessing EVERYTHING.

Sassy with 252 HP / 200 Atk / 56 SpD is the standard Curse set. Personally I'd take 60-100 EVs in attack and move them to S.def for better bulk. Or into Def if your wanting to prioritize wish or BP passing.

Now I see you mentioned keeping it at a Sassy nature. I was told giving it a Careful nature would give it a decent SpD and Def Stat even if you didn't EV them to max. I'll give this a shot since I've bred about 30 Eevees and bound to have at least one with a Sassy nature that knows Curse.

Sassy = +S.def -Speed
Careful = +S.def -S.atk

For umby either is good, in the end they do the same as curse lowers speed and he doesn't use s.atks.

Sassy has its merits; makes it slower, so Curse has a better chance of getting the boost (lol, as if it wasn't slow enough). Either one is fine though. I decided to make mine Sassy with 0 speed IVs.

I'd use sassy, so with curse you will go last almost all the time to power up payback.

I've used it in WiFi before, and it's a real killer. However, it's not very good against walls unless you use wish then start using curse like your life depended on it.
My bf LOVES it.