Before doing it, please take some time to check the existing issues and make sure what you are about to report isn't already reported by another person. In case someone else reported the same problem before or a similar one, and you have more details about it, you can write a comment in the issue page... a little more help can make a huge difference!
In order to write a new issue, take into account these few tips to make it easy to read and comprehend:
Try to use a descriptive and to-the-point title.
It's a good idea to include some sections -in case they're needed- such as: steps to reproduce the bug, expected behaviour/response, actual response or screenshots.
Also it could be helpful to provide your operating system, browser version and installed plugins.