There are several posts in the forum that cover this said topic, and I think it's about time someone (possibly me) put together a comprehensive tutorial on creating a skin from scratch for DNN4 and DNN5.
I'd recommend having a look at the skinning guide: http://www.dotnetnuke.com/Support/Docu... although this is more focused on DNN4 it gives you a good overview of the process with regards to using DNN skin tokens.
Then have a look at this post, as it covers some DNN5 related skinning options: http://www.dotnetnuke.com/Community/F...
Once you've had a read over these, I'd recommend looking at the MinimalExtropy skin files that comes with Dotnetnuke, the files can be found in your DNN installation int he following directory Portals/_default/Skins/MinimalExtropy. This is probably going to give you the best look at how to build a skin as it contains all the files required for a skin pack.
When I'm working on a skin pack I always follow this kind of path:
1. Design
2. Build of HTML and CSS
3. Removal of content elements such as banners, text, navigation, user controls
4. Adding of skin tokens and panes
5. Styling of skin tokens
6. Testing the skin on a local server and refining the styles
7. Creation of containers
8. Packaging up of skin files
If you're working locally, I'd recommend installing a copy of DNN to your machine as it helps to speed things up, and you can use the Windows Platform Installer to do this: http://www.microsoft.com/web/download...
I think if you read through the skinning guide, forum post and have a look at the skin this should give you a good grounding on the skinning process, and if you have any questions you can usually get answers on twitter, or through the forum.
Hope this helps you out :)
Rick.