This guide is boosting only from level 10+. To have 60's in a reasonable time level 80 Frost Mage/Protection Paladin recommended for boosting. Good luck and enjoy this guide.
Levels 1-10
Here you would want to group up with your friend and just quest. You should be level 10 when you reach the next questhub (Goldshire for example). You could use Zygors guide trial for this as it gives you guide to level 1-13.
Zygor Guides
Levels 10-20
As Alliance you would want to go to Deadmines now and stay there untill you are 20. Horde can go to SFK or Deadmines. If your booster is "too fast" and you get the "You have entered too many instances recently" message there is a way to get rid of that. You can find that part at the end of the guide.
Level 20-40
Scarlet Monastery. You will be staying here untill level 40. You would want to do Scarlet Monastery: Cathedral (You need the key for this!) and/or Scarlet Monastery: Armory. I find Cathedral more faster and since you can do it in one pull (more about this in the end of the guide also) it is great xp/h. The instance limit bypassing will come here even more handier since Scarlet Monastery runs dont take that much time when being boosted.
Level 40-45
Zul'Farrak. This instance gets you from level 40 to 45 in no time at all. You get about 1,5 levels/run here if you clear the whole instance. Remember to pick up 3 Zul'Farrak quests from Gadgetzan as they give you loads of xp. The best thing about this instance is that the booster and you can both mount making the pulling much faster.
Level 45-60
Alright you are almost there! Now its time for Scholomance. This instance gives crazy xp and is rather quick to do. Paladin booster is best here because some of the mobs are immune to all spell schools except holy. Also melee will work against the mobs. If you cant however get a melee class to boost you the instance is still pretty nice xp/h even though it is only advisable to kill all the mobs untill Rattlegore. After you have killed him you can run out
and reset.
Optional
You can also go and do Hellfire Ramparts after you ding 58. It does give better xp than Scholomance and even if you dont have bought TBC yet you can activate the TBC trial on your account.
Tips & Tricks
Bypassing the instance limit. Credit goes yo Yo Dawg for this.
1. Give leader to one of the toons that are getting boosted (If they didnt have already)
2. Log out on the booster (Inside the instance)
3. Run out of the instance with toons and reset as you would normally. (At this part group should look like this: Disconnected booster, Toon with lead, Toon without lead.)
4. Leave the group with both toons.
5. Log back on main, invite the toons to group and have toons enter instance.
Pulling Scarlet Monastery: Cathedral with one pull.
1. Kill the mobs in the start so the toons can come to the doorway.
2. Go straight to the Cathedral door without pulling any mobs.
3. Attack the boss that is near the altar (You will aggro every mob inside the cathedral by doing this.)
4. Kill the boss and wait for him to get resurrected.
5. Run outside the cathedral with all the mobs on you. (This will aggro every mob inside the courtyard.)
Getting inside Scholomance without the key.
1. Duel with the toon who has lowest hp
2. Win the toon in duel
3. Toon runs up to the roof (There is a way from inside the castle to the roof)
4. Toon jumps down and dies from fall damage
5. Toon runs for the corpse but instead of resurrecting at the corpse he goes through the instance door and resurrects through the wall
6. Open the door for others (The lever looks like a torch)
Final words
I hope you enjoyed this guide and that it helped you. With this you should get 60's with about a day /played time maybe even less. Please leave some constructive criticism and tell me what could be improved and such. Thank you for reading this and have fun leveling your 60's!