Teleportation Guide by Lampir.
What to do in steps:
1) Buy a Rune (Arcane Spells) or Leaf Holder (Nature Spells). These can be found in Sorcerous Sundries in Balder's Gate and on the bazaar in Sshamath.
2) Go to a location you would like to set as a point you'd like to teleport to in the future. (Druids, this must be in a area set to 'Natural' and ideally beside a tree.)
3) Cast the level 0 cantrip (mark rune / leaf) onto the Rune/Leaf Holder. It costs 50 gold to set a location. When you move the Rune/Leaf Holder in your inventory you will see it now has a new title of the location you set it to. Casting the cantrip on the Rune/Leaf Holder again will un-mark it, letting you reuse the Rune/Leaf Holder in a new location.
4) Now, go to a new location and cast one of your teleportation spells onto the Rune/Leaf Holder. If the spell can take other people with you, you must have them in your party to move them with the spell. Teleportation spells range from kinda expensive to REALLY expensive and have a one hour cooldown between use. This cooldown applies to all teleportation spells. The benefit of expensive spells is that they decrease the DC or can take more people. (Tree stride is a solo teleport for example)
Arcane vs Natural Teleports
Arcane teleportation spells are more complicated to pull off, but the trade off is they are more versatile. A Rune can be set virtually anywhere provided the area is not warded against teleportation. There are concentration checks and % chances of you messing up and either taking damage or ending up in a completely different area. ((If a wizard would like to rewrite this to explain better I'm just a druid player here))
Natural teleportation spells are pretty straight forward, less risky and more predictable. A Leaf Holder can only be set in a natural area. I believe (have not tested) that you again have to be in a natural area to teleport to a marked location. Tree Stride requires you to travel from one tree into another. Travel by Plant is much more versatile. As of this time I'm unaware of any location in the Underdark that is a natural area. There is no concentration check and no % chance of messing up. However if the tree you marked has been killed, you will die when you Stride to it.
Another drawback of Arcane teleportation is that they are inhibited by Dimensional Locks and Anchors, while natural ones are not. Shadow Walking is an arcane exception to this, though it is also quite dangerous.
Clerics and Favored Souls have access to the Word of Recall Spell which can store one location to teleport to and does not work with runes. Unlike the arcane teleports Word of Recall does not have a DC. It is affected by Dimensional Locks and Anchors however. Aditionally the Travel domain has the arcane spells: Teleport, Dimension Door and Greater Teleport. These work as they do for arcanes. (note: this is not the original NWN2 travel domain!)
A journey into the Shadow is dangerous by any measure. Most who risk prolonged exposure are never heard from again, driven mad or killed by its native inhabitants. For those powerfull or desperate enough the Shadow provides a way of alternative transport, unimpeded by Dimensional Locks and Anchors.