The Screen Ruler app was recently featured by AddictiveTips.com in a short video tutorial. The video gives a nice overview on how to get started with the ruler on Windows.
You can find it on YouTube at https://youtu.be/UPiqYMfwbjY.

Previously, AddictiveTips already published a review of Screen Ruler on their website at https://www.addictivetips.com/windows-tips/add-a-ruler-to-the-screen-on-windows-10/.