In addition to pen and paper, there are now many tools that support the development of prototypes. There are three practical helpers from our point of view in this case.
Axure: Axure can be used, for example, to simulate detailed interactions between the user and the system interface.
Balsamiq: At emineo, we like to use Balsamiq when we do not want to be distracted by user interactions and styling. Thus, it comes closest to the oldest (and depending on personal opinion, best) combination of paper and pen.
Figma: This tool is the defacto standard UI designer today (as of August 2022) and impresses with its visual styling. However, it is usually more time-consuming due to the high level of detail needed. We tend to use Figma towards the end of the design process.