Okay OP let me give you a small overview on this stuff.
IVs are set values that the pokemon is born with or in the case of it being caught are randomly generated. They give relevant boosts to stats just like EV's. 31 is the max one can have in a stat and they get passed over by parents during breeding (3 get carried over at random, 5 if one of the parents is holding a destiny knot).
EV's work differently in that you can control them. 4 points into a stat = +1 to that stat. A pokemon can have a maximum of 500 EV points and a maximum of 252 in any stat because programming (it used to be 255). Super training gives you an easy way to keep track of them as well as raise them since you just select the stat you want and play. Normal EV training requires you to go find hordes of pokemon (or just random wild pokemon work too but its slower) and have you kill them while your pokemon is holding the stat you want to raise appropirate power item (it gives a +4 bonus to the EVs you receive for killing the mons in the horde). You can also go Macho brace instead although its slower, Macho brace will double the points received (so killing zubats with macho brace on will give you +2 speed instead of +1).
Basically, you ideally want to get yourself some Dittos from the friend safari, breed some IV's into baby Eevees that have the right egg moves, then, once you get a good IV Eevee with all of the above and a good nature, you can start EV training. Sounds tedious I know, but if you don't plan to go super hardcore smogon and just casual online battling, just EV training will probably serve you well enough.
JIC you're interested, theres an IV checker in the Pokemon Center in Klioude city. He's the purple guy next to the TV. He will tell you your Sylveons worse IV's (if it has any) as well as its best IVs.
