Its done with Nogg-It or the Worldediting Tool (which i prefer) on "MM*ed", but they don't work on Vista. i wouldn't try to make skypaths as you are reported/banned very fast (other players see you floating) Better remove/flatten some Rocks/Buildings so you can hide in or stand on, make a steep hill not-so steep, create windows to begin a pyro before the enemy even sees you, build a underway-tunnel-system to flags, towers and so on, be creative!
Very funny is it also for some Private Servers PvP-Token-Zone (like the Ring of Trials in Nagrand for Team-Atom
WoW) although you will get faster reported/fucked because there are much more gms on Private Servers and noone likes a pyro from below earth