As climate change is intensifying the frequency and severity of floods around the globe, adaptation is increasingly vital. Besides structural measures to mitigate flood risk, non-structural measures are known to be highly effective and low-cost. Such non-structural measures include Flood Early Warning Systems (FEWS). Effective warning creation and dissemination are crucial to successful FEWS. Despite extensive bodies of research that cross the boundaries between disciplines and application domains, systematic understanding of the detailed aspects contributing to the effectiveness of flood warnings is lacking. We systematically review the state-of-the-art in risk perception and warning communication present in academic (and grey) literature for FEWS. We focus on the elements of risk warnings specifically, rather than reviewing the topic of risk communication in general. We start with exploring how personal attributes affect individual risk perception related to flood warnings. We then deconstruct flood warnings into three basic components: content, format and dissemination channel. Most importantly, we found 21 individual elements (options) for these components, each associated with varying levels of support for their effectiveness in literature. Important caveats were identified, such as a lack of research into the speech format and SMS channel. We then describe and visualise the warning creation process, providing a framework for guidance. Accelerating technological advancement necessitates continued research into the effectiveness of novel formats and channels, rendering the currently most widely supported and researched elements increasingly obsolete. Further research is needed to explore the complex interplay between elements, i.e., how do different combinations impact effectiveness? Finally, little is known about the transferability of our findings to Africa, Asia and South America, as industrialised countries dominate the research. We hope our findings will contribute to improved understanding, and support the practice of creating effective flood warnings.