Naming the theme
Pick a name for the theme, which will be the name of the theme's style sheet file. Check in the themes directory of the Tiki installation to make sure there isn't already a theme there with the same name (for example, themes/newtheme/css/newtheme.css file).
There is no authoritative register of Tiki theme names but, if you intend to share or distribute your theme, it would be good to check at themes.tiki.org and in the standard Tiki download package to make sure there won't be a name conflict.
Choosing the way to do it
There are at least three ways to make or adapt a theme for Tiki - a simple way, a way that's not quite as simple, and a way that's a little more involved.