If you have one existing account, and want to get into boxing, call that account A.
Also, if you're brand new to the game, you could go with this same setup.

(A) is the parent account.
It will sent RAF invites to both B and C.
You will use the email code, to create both B and C.
And use retail/digital codes to update them to full accounts (at least as far as WotLK, for the level 80 cap).

Then B sends an RAF invite to D.
And C sends an RAF invite to E.
These are the same process as above.

Giving you:
A > B > D
A > C > E

If your existing account is A, choose the best "booster" character.
Which is generally your best equipped character, who is at or close to maximum level.
Many prefer a Prot/Ret Paladin, but a Death Knight, Feral Druid, or other tank type works well, and any AoE class works somewhat.
At lower levels... boosting to 70th or whatever, almost any class will work very well.

The process is...
Group everyone together.
Have A, go and kill stuff, with BCDE on follow.
Until 10th, when you can do instances.
Park the lowbies at zin (zone in), clear a bunch or pull half the instance to near zin, kill them.
Move everyone up, to the next point, and repeat the process.

If you don't have an existing account, and they're all new.
Same process, except have a good booster, on Account A, as one of the first five that you level.

The reason for the double chain... with A referring both B and C is...
Double granted levels.
A boosts BCDE, four toons each.
D/E each give two free toons (granted levels) to B/C.
B/C each do the same to A.
Then B/C each give one more free toon (the granted levels cascade), to A.
Resulting in:
A Booster +6 Granted.
B/D 4x Boosted + 2x Granted
C/E 4x Boosted

You only actually gain 39 levels from granted levels.
39x2 = 78.
So you'll be able to give 78 levels, from two toons who were boosted/leveled from 1st to 80th, to a toon on the parent account.
This means, you make a toon on A, and level it until its as close to 3rd as possible but has not reached 3rd level; if you accidently get to 3rd, do the same thing but end up close to 4th.
You give 78 levels (39 from toon A and 39 from toon B) to the level 2.
Who is now level 80, and the same percentage into their level... so almost 81st.