I've been hunting skins down for other reasons when it hit me that there's some impressive skins for Unreal1 monsters in the ut2004 Invasion. Like this one for the Warlords:

I even went looking for more and found a few dozen good ones, like this wicked looking thing for the Krall:

Thing is I've tried skinning monsters on maps before with mixed results. One time it worked great...but I haven't managed to do it again.
I went into monster properties, a specific monster for a specific skin, went under display, and under skins put the skin in I wanted to use. It worked, a variant Skaarj that is fairly amazing. However, when I tried it for the Warlord it didn't work even though I did the exact same thing.
Take me back to basics. How do you skin monsters?