The Skinny on Skins
When Rainmeter first came out, configuration of the skins required manual editing of the .INI files, so it was more popularly used by people with coding knowledge or by the more technically minded. With the 1.3 release, more of the skin developers have offered a GUI-based configuration tool. Gnometer, one of the default skins that comes with the Rainmeter installation, has a nice GUI for configuring the look, changing the RSS feeds, etc.
Rainmeter was initially available at Customize.org, but that site lacks updates and the most recent additions are at least a year old. These days, the site of choice is Deviant Art. DA is a decade-old site that claims to be "the largest online social network for artists and art enthusiasts with over 13 million registered members, attracting 35 million unique visitors per month."
Rainmeter skins are just one portion of DA, which mostly has a graphics art theme, with subjects ranging from literature to paper craft. The Rainmeter section lets you browse by most recent additions, so you can keep up with what's new.
One thing you have to get used to with DA's skin collection is that people like to bundle existing skins together into a package along with their own mods and additions, ranging from new skins to adding art and wallpapers. Other folks make them entirely from scratch.
If the skin does not come with a GUI to configure it, you can edit a skin by right clicking on it and clicking on Edit Skin. Since these are often transparent skins, you may get the Windows desktop instead of the skin, so keep clicking. Selecting Edit Skin will bring up the .INI file, and if it's well written, there will be comments in there telling you how to make changes. For example, the Weather skin that I use has this:
;-- LOCATION -- Go to http://www.weather.com/ search for your city.
;when in your city page, Copy the 8 digit code at the URL in the address bar | Paste the location code below.
;-- UNIT -- Choose either m, for metric (Celsius/km), or f, for imperial (Fahrenheit/mi) | Choose the unit below.
Pretty straightforward, right?