First off I want to state that I am no master of Rainmeter. The beginning of this will mostly cover things in the help file but with examples and walkthroughs of what it does and how to apply. Most places I've come across just tell you to jump right into someone else's skin and just start editing around but that isn't the method I really enjoy. Not to mention that method just makes the ini files a total mess. People edit around code they don't understand and get the skin how they want it ..maybe adding code from other skins and then someone else gets their skin and edits that around even more code they might not need and the process can keep repeating over and over till you have a total ugly coding nightmare of slop. As I write these I will also be learning again 
Tutorial One [1] - Getting started with basic background and image placing
Tutorial Two [2] - Measure Disk Space
Tutorial Three [3] - CPU, Memory, Swap . . . Oh my!
Tutorial Four [4] - Net In, Net Out, Histogram and Line
Tutorial Five [5] - Variables, Hiding/Showing Meters, Hotspots, and Buttons