Okay. Here's how:
First, make a small sector inside your level and give it a sector tag of 1. This is the CALLING SECTOR, the sector that will teleport you if you stand in it. Set the sector effect to either 974 or 975. If you choose 974, the teleporter will work anywhere in the sector. If you choose 975, you will have to step on the floor for it to work.
Now, make another small sector OUTSIDE OF THE LEVEL. This is called a "control sector". Give one of the linedefs a tag of 1, and set the Linedef Action to either 97 or 98. If you choose 97, the teleporter can be used as many times as you like within the level. If you choose 98, the teleporter will only work once.
Now, choose another linedef in the control sector, give it a tag of 2, and set the Linedef action to 104. Got all that? Now, make another small sector INSIDE the level, at the place you want to be teleported to, give the sector a tag of 2, and place a Teleport Destination Thing inside of it.
If you've done all that correctly, your teleporter should work. For future reference, note that the sector tags don't have to be 1 and 2. They can be anything as long as the linedef with type 97 or 98 has a tag equal to the tag of the calling sector, and the linedef with type 104 has a tag equal to the tag of the destination sector. Also, nothing else in your level should have the same tags as the tags you use for your teleporter, or you may experience problems.
And by the way, this IS all in the Wiki...if you already searched there, you can't have looked properly