Decision support tools are popular outputs from academic projects aiming for societal and policy impact in the fields of climate and environmental research. A considerable number of these tools have been made, and there are numerous reviews and lists of existing tools, as well as some papers evaluating these tools. However, there is a lack of evidence that these tools reach those they are intended for, work well or are used to make policy decisions; as such there is a danger that they can be perceived as greenwashing. Surprisingly there is little advice on how to make a good decision support tool. The purpose of this review is to reduce the 'unknown unknowns' for people wanting to develop and build this kind of tool. We provide an overview of different types of decision support tools with relevant climate/environmental examples for each. We then cover the key considerations for designing and building better tools, from a developer's perspective. Our hope is that this supports more effective decision support tools to be made, and eases the process for those who are new to this area. All referenced resources are open access, and archived links have been provided where relevant and possible.