Teleporters are a general kind of linedef executor, and probably the one people will use the most. You have the sector that you want to teleport from, which I'll call the teleporter. There is your control sector, which has one linedef with a type of "Trigger Linedef Executor: Repeatedly" (may differ based on editor),which is your trigger linedef, and another control linedef on the same control sector (THIS IS IMPORTANT) that has a type of "Teleport player to tagged sector", this is your teleporter linedef. Finally, you have the sector you want to teleport to, called the target sector.
Now, make your control sector as usual. Have each linedef marked as I told you above. The trigger linedef should be tagged to the teleporter (the originating sector) and the teleporter linedef should be tagged to the target sector. Now, one last thing. Your target sector should have a thing in it. This thing is the teleport destination. If you are using Doom Builder, chances are this isn't in your listing (Partly my fault :p), so just put thing number 5003 in the thing properties box. This thing determines where you will end up in that sector after you teleport. X, Y, Z, and even directiona re determined by this.
Hopefully, that's detailed enough for you. If it's not, crack open a map that uses it, and you can see for yourself.