" Usually kept high up on the reef for strong water flow and highest possible lighting. Xenia will reproduce in the tank by attaching its stalk against adjacent surfaces it contacts and splitting into two colonies. In this way, Xenia colonies tend to 'walk' in the direction that water movement bends their stocks, so you may want to consider this in your placement. Xenia can usually be coaxed to grow up the back glass of the tank and forms a nice background display."
(from
http://www.reefcorner.com/SpecimenSh...ting_xenia.htm)
Just some ideas of where you want to place them. As for "how," for something that grows like this, I'd think you'd just put it anywhere that it will stay more or less still as far as water movement is concerned, and let it grow and root itself.