Whilst new patents and announcements advertise the technical availability of foldable displays, which are capable to be folded to some extent, there is still a lack of fundamental and applied understanding of how to model, to design, and to prototype graphical user interfaces for these devices before actually implementing them. Without waiting for their off-the-shelf availability and without being tied to any physical foldable mechanism, Flecto defines a model, an associated notation, and a supporting software for prototyping graphical user interfaces running on foldable displays, such as foldable smartphone or assemblies of foldable surfaces. For this purpose, we use an extended notation of the Yoshizawa-Randlett diagramming system, used to describe the folds of origami models, to characterize a foldable display and define possible interactive actions based on its folding operations. A guiding method for rapidly prototyping foldable user interfaces is devised and supported by Flecto, a design environment where foldable user interfaces are simulated in 3D environment instead of in physical reality. We report on a case study to demonstrate Flecto in action and we gather the feedback from users on Flecto, using Microsoft Product Reaction Cards. CCS Concepts: • Human-centered computing → Displays and imagers; Interactive systems and tools; Graphical user interfaces; Virtual reality; Ubiquitous and mobile computing design and evaluation methods; • Software and its engineering → Integrated and visual development environments; Rapid application development.