When making skins with Artisteer you need to make sure you start with a skin from /Data/skins not /Data/Sites/1/skins since those never get updated during upgrades. So fixes and skin updates only happen in /Data/skins, so that is where to get the latest skin as a starting point.
Which skin you use as a starting point should be determined by the criteria in the article, not by which .artx file you start with.
Artisteer 3 is still a release candidate. There have been several issues with Artisteer 3 skins reported since the 126.96.36.199 release of mojoPortal, and these have been fixed in the source code repository so it may work better for you after the next release. There will be a new release of mojoPortal before the end of the month.
Note also that the .artx files I've made available have been upgraded over time from previous versions of Artisteer and that upgrade conversion may mean they have differences compared with a design created from scratch in the latest Artisteer. I made those available as a courtesy, but I don't consider myself a good designer and don't really expect many people to want to use those. I think just about anyone could start from scratch in Artisteer and make better designs than me.
Starting from scratch is probably cleaner and less problematic than using those .artx files which have gone through many version conversions. There can be anomalies in upgraded .artx files that are not there in new designs.
What is the best approach to make skins?
Whatever works for you is the best approach. CSS gurus may have different approaches and the system is very flexible. Starting with an existing skin is a good idea for many/most people.
Hope that helps,