Digital modeling software is essential in engineering, where the fabrication of mechanical components is costly in both time and material, but their design can be handled efficiently via Computer Aided Design software. Similarly for origami, crease pattern drawing software has enabled the design of countless new models. However, software to model step-by-step flat-folding procedures, for example, to fold models virtually or automate diagramming, remains elusive. Existing origami flat-folding simulators often restrict to very simple folding operations like pureland mountain and valley folds that cannot generate most origami models. In this lecture, I will discuss what we need to build expressive flat-folding origami simulator software, and current work in that direction.