First what you have to do is go to:
[Only admins are allowed to see this link]

And download quice, this is a mangos database editor which is EXTREMELY helpful
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Then Make a custom creature by, well going into the database and
creating it. If your not sure how to make a custom creature look up a
guide. (Your going to want to set the faction for alliance and horde to
35)

After this go to the quest tab and click search, at the bottom there will be a "New Quest" Button.
[Only admins are allowed to see this link]

From there you will be on "Quest Part 1"

There are the fields you will want to worry about.

Title: (Name of the quest)
Details: (What you want it to show when you are accepting the quest
OfferRewardText: (Here put your same as you put in the details)
Entry: (900000+)
MinLevel: (What level you have to be to take the teleport)
QuestLevel: (If its at 1 and on your server everyone is level 80 then
its going to show up as a low level quest, so you want to put it at the
max level for your server)
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Now head to "Quest Part 2"

And there will be only 3 fields for this that you have to fill out:

Method: Change to 0
Complete Script: Something 50000+
SpecialFlags: (Change to 1 if you want it to be repeatable teleport)

Picture: [Only admins are allowed to see this link]
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Now head to "Questgivers"

This is simple, click "Add" Then find the creature you want to be the
teleporter, do the exact same for the "Quest Taker" tab also
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Now head to the tab "Complete Script"

At the bottom those are all the fields you want to fill out

-Before you enter any of this data, you want to go to the spot you want the teleport to go to, and type .gps
This will give you the co-ordinates you have to enter for some of the feilds

ID: The id that you put for complete script in "Quest Part 2"
Delay: 0 (If you want it to be instant teleport)
Command: 6 (This is the teleport command)
Datalong: Map Id (You get this from .gps)
Datalong2: Leave at 0
X: (The X co-ordinate you got from .gps)
Y: (The Y co-ordinate you got from .gps)
Z: (The z co-ordinate you got from .gps)
O: (The Orientation co-ordinate you got from .gps)
Dataint: Leave Blank
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Then go to SQL-Script Tab and click "Execute Script"

Once it says its successful then restart your mangos private server and add the npc which will be able to teleport you [Only admins are allowed to see this image]