How Do I Give A Good EV Spread In Competitive Pokemon

EVs spread can be changed in order to optimize your pokemon against his main counters and/or other opponents. I begin explanations with basics :

• 4 EVs increase of 1 point the aimed stat, also you can give a max of 252 EVs since gen 6 (it was a max of 255 EVs before, but I recommend you to read more about it on that Bulbapedia page as it would be too long to explain and off topic) to a same stat and 510 EVs at total (so 2 EVs as a margin)
• Usually, we tend to maximize 2 stats in order to be faster and attack more harder (in most sweeper cases)

But there are few pokemons which we can adapt these EVs spread, in order to be able to survive against few opponents particular attack, so this pokemon could :
- survive and win his fight against his counter
and/or
- become a better check against other usual opponent which he can't survive at 2 attack from him if you would have maximize 2 stats

Let's take an example : here is the Smogon guide usage of Venusaur in OU.
As you can see, Venusaur EVs spread are really weird, with 252 HP,172 Def,68 SpD and 16 Speed, but if you keep reading, you can understand why EVs are like that :

The EV spread listed allows Venusaur to survive two Dark Pulses from Choice Specs Greninja after Stealth Rock while outspeeding maximum Speed Mega Mawile. The rest is dumped into Defense to make Mega Venusaur a worthwhile Mega Mawile, Kartana, and Tapu Bulu check.

Just don't forget that you can always choose different ways to use your pokemon, as Smogon is a really good databank for beginners, because it can surprise your opponent if you spread differently your pokemon (in both senses of the term). And there isn't a unique moveset suitable for every pokemon, so it's the same for EVs spread.