Xenia is very hardy when trimming it. There really is no way to do it wrong. Just be careful with the smell!! It will make you puke if you take a huge whiff.
Also, I got two small clippings last year and now I've had to trim it 5 times already. I cut back on my Iodine treatment and it seems to have slowed down the growth a bit. The good news is that it doesn't seem to bother the other corals.