If its an APK, install it with a file manager, and apply it in the theme Chooser app in the app drawer. Not all themes are flashable. If its an apk, install it. If its a zip, it may be flashable, but always make a copy and unzip it first to make sure they didn't just zip up the apk with some extras.
Tapatalkin it from my IHO CM7.1'd, MIUI Themed Optimus V