The most important step after installing the program on your computer is to calibrate the display(s). This is important, because different screens come with different DPI (dots per inch) resolution, therefore you will not get the exact result when measuring things in units other than pixels (the pixel is device-specific and doesn't get affected by the DPI resolution of the display).
To calibrate the displays, right click on the ruler (open the context menu) and select "Preferences", then click over the "Displays" tab. A step-by-step example is available in the "Calibration" section of this document.
Then you may want to take a look on the different menus. A lot of options are available in this version (compared to the previous versions) and some menu items and shortcuts are different.
The biggest new feature, I believe, is the Flying Figure concept. When you start the program, you will see one "inner ruler" that you can move, resize and rotate. This is just one of the figures available (the others, however, are just figures, not rulers). Look at the "Flying Figure" menu to choose a figure (or use the keys from 1 to 7 and 0 on your keyboard).
- To move the flying figure -- click inside the figure and drag;
- to resize/rotate figure: click on the control point (labeled as 1 and 2) and drag to the desired position;
- to avoid rotation while resizing: press and hold the Command (Mac) or Alt (Windows) key while dragging the control point;
- to rotate by 45 degrees: press and hold down the Shift key while dragging a control point;
- to get the full description on flying figure usage, read the Flying Figures section of this guide.
For a list of "hidden" keyboard shortcuts (functionality accessible only with key combinations) -- read the Hidden Keys section.
To learn how to define your own units (with step-by-step example) -- read the Units section of this document.
If you need help with the activation (to unlock the program after the trial period expiration) -- there are some guidelines in the "Activation" section.
To understand how important one single pixel could be and how it affects the precision of the measurements -- read the "pixel tale". In this section you'll find more about the difference between 0 and 1 and why there could be "strange" differences ruler, guides and figure readings. (This section, along with the "Calibration" guidelines will give you better idea on how precise the ruler is and why)
In addition, there is a quick guide on how to use guides. (And what exactly the size between two guides is).