The purpose of visual media is to convey information, ideas, concepts and emotions, and for this reason, the effectiveness of visual media can be assessed by how much it captures the attention and engages with its audience. Eye movement patterns have long been recognised as providing valuable insights into the cognitive processes that underlie attention, learning and memory, and as such they may shed light on how viewers engage with visual media. The process of measuring and analysing the movements of a person's eyes is called eye-gaze tracking, which is a powerful tool with a broad range of applications, not only in studying how people interact with visual content, but also in domains such as healthcare, driving, gaming, and many others. Thanks to advancements in technology, modern eye-gaze trackers have evolved into much less intrusive and more comfortable devices than their scary predecessors. This has also worked in their favour in making eye-gaze trackers, whether screen-based, head-mounted, or embedded within VR headsets, more accessible and easy to use. In view of the increasing popularity in using eye-gaze tracking to study attention and engagement, this tutorial aims to explore this technology from different angles, including its development over the years, its technical workings, the metrics that may be used to quantify visual attention, and several application domains. The tutorial will provide an overview of eye-gaze tracking, covering the state-of-the-art, the different types of technologies in use, the key eye movements and gaze tracking metrics in quantifying attention, the factors to consider when choosing an eye-gaze tracking device, and several eye-gaze tracking applications. This tutorial is intended for all researchers, with or without a background in the domain, who are interested in learning more about eye-gaze tracking and its use in assessing visual attention.