It pretty much depends on the Pokemon itself on whom you want to give EVs.
Say you have a Weavile. What's it's sole purpose in life? To hit hard and fast. So that means you should invest completely on it's Attack and Speed and the reamining EVs can be given to HP so that it can survive an attack a little better.
Similarly an Electivire also has to hit strong, but it doesn't have the kind of single-minded movepool and stats like Weavile. You can go diverse with it and add both special and physical moves. In this case, one should consider splitting EVs into Attack and Special Attack and usually go max with the speed.
EVs must be given after taking into consideration:
>what your Pokemon needs to be
>what your team needs it to be
>what it needs to cover
>what it can do
So it's not necessary to run full EVs on the highest stat. Example - A Gyarados' best asset is it's Attack, but it is very viable to run a bulky set (investing EVs into HP and Defenses primarily) due to it's good defensive typing and movepool.
One might not get this in a day, but that's just how complicating EV training can get.
As for after the EV training, you don't have to feed it rare candies, just level it up like normal, and it should still get all it's EVs.
Hope I helped!
