Pokecenter Poke Guides in General

Basic Knowledge
A pokemon only has 510 evs to waist. You can speard the evs how ever you want. Only 252 evs can be put into a stat to maximise that stat. But before any stat can be maximised you have to check its IVs.
Checking IVs
To check a pokemon's IVs is fast and easy. What you do is you ask a friend to battle you in a LV100 single, but you don't actually battle. You go here to check IVs http://www.serebii.net/games/iv-calcdp.shtml and fallow the instructions.
Items that Help
Protein +10 evs on Atk
Hp up +10 evs on Hp
Iron +10 evs on Def
Calcium +10 evs on Sp.Atk
Zink +10 evs on Sp.Def
Carbos +10 evs on Spd
You can give your pokemon up to 10 of each.
-Macho Brace doubles the evs you get from battle.
-Power Waight +4 evs on Hp from battle
-Power Bracer +4 evs on Atk from battle
-Power Belt +4 evs on Def from battle
-Power Lens +4 evs on Sp.Atk from battle
-Power Band +4 evs on Sp.Def from battle
-Power Anklet +4 evs on Spd from battle

The PKRS Factor
PKRS is a virus, a helpful virus. The only use for PKRS is to speed up the EV training process by doubling all evs you get from battle. Nothing more about it, but VERY HELPFUL.
What pokemon to battle
Every pokemon gives out evs. Here is the list of pokemon i battle to gain evs.
-Bidoof-give out +1 evs on Hp
-Shinx-give out +1 evs on Atk
-Geodude, Onix- give out +1 evs on Def
-Gasley- give out +1 evs on Sp.Atk
-Tentacruel- give out +2 evs on Sp.Def
-Starley- give out +1 evs on Spd

Combining Everything
What i do is start by giving them the appropriate item for exm.
Cresselia w/standerd wall
EVs: 252 Hp, 252 Def, and 6 Sp.Atk
Step 1- I would use 10 Hp up and 10 Iron.
Step 2- Give it PKRS.
Step 3- Give it Power Waight.
Step 4- Battle Bidoof and with all that Bidoof gives out +10 evs on Hp and since you gave Cresselia Hp up she already has waisted 100 evs for Hp. Only battle 15 Bidoof with Power Waight on then remove and battle 1 more to make 252 evs.
Step 5- Give it the Power Belt.
Step 6- Battle Onix, and Geodude in Orburge cave for def. They both give out +10 EVs on def if have the power belt on and the pokemon has PKRS. Agian only battle 15 with the item on then take it off, and battle one more.
Step 7- Give it one Calcium.

Natures and Effects
Most Natures have a cause and an effect, but there are some that are neutral. Here are the Natures.
-Adamant 10% +Atk, 10% -Sp.Atk
-Brave 10% +Atk, 10% -Spd
-Lonely 10% +Atk, 10% -Def
-Naughty 10% +Atk, 10% -Sp.Def
-Modest 10% +Sp.Atk/-Atk
-Mild 10% +Sp.Atk/-Def
-Quiet 10% +Sp.Atk/-Spd
-Rash 10% +Sp.Atk/-Sp.Def
-Hasty 10% +Spd/-Def
-Jolly 10% +Spd/-Sp.Atk
-Naive 10% +Spd/-Sp.Def
-Timid 10% +Spd/-Atk
-Bold 10% +Def/-Atk
-Impish 10% +Def/-Sp.Atk
-Lax 10% +Def/-Sp.Def
-Relaxed 10% +Def/-Spd
-Calm 10% +Sp.Def/-Atk
-Careful 10% +Sp.Def/-Sp.Atk
-Gentle 10% +Sp.Def/-Def
-Sassy 10% +Sp.Def/-Spd
-Bashful No Effects
-Docile No Effects
-Hardy No Effects
-Quirky No Effects

First of all cloning works on all games
#1: Have at least two pokemom in ur party and the one you want to clone last in ur party
#2: Offer the one you want to clone for something nobody has like Arceus lv 9 and under
#3: Look at the clock at the buttom right of the screen when it is saving (shounld go around from 7-8 times) you want to wait until it get to about 7.45 to about 8 you shut off ur DS and turn it back on ur goal is to turn off ur DS one bar before ur pokemom goes up if it doesn't say corrupted data you did it wrong and have to start over. ur pokemom is at risk when you do it wrong but not a big risk when you ask for Arceus lv 9 and under so don't stress urself but if you did it right you collect ur prize