Hey Harriet,

You would need to create appropriate HTML markup in the layout.master, write up CSS files for each of mojoPortal's features (blog, forums, etc.) that you are going to use on the site, and you might need to configure the theme.skin file such that mojo will generate the correct markup to match whatever css you used for various things (pagers, menus, etc.), which might require some study for somebody who isn't familiar with it.

If you're looking for help, i7MEDIA can convert a current site to a new, fully hand-crafted and custom built skin. We can also convert PSD's or HTML Templates to mojoPortal skins.

Hope this helps,
-Isaac

Note also that creating a mojoPortal skin using Artisteer 3 required some manual steps making it more difficult, whereas with the latest version of Artisteer (4.1) we have a free plugin to directly export a mojoPortal skin from Artisteer. 

Artisteer is a web design tool for people who may have design skills but not web design skills (ie little or no knowledge of html or css), without such a tool then web design requires the actual web design skills with css and html javascript browser dev tools etc.

A really good web designer probably could make a new skin from scratch in an afternoon or a day whereas one with less skill or experience may take a good deal longer due to more trial and error on the way to getting results.