Ask HN: How to mock up UX flows?

I'd like to be able to build and test user experience flows/different user interface layouts, cheaply and quickly.

I need to be able to: - build and test and idea quickly; see what a user does, and then iterate - see how it works with different screen sizes and aspect ratios

I would prefer: - to do things in a low-fidelity way. (If I have to open up photoshop for a UX mockup, I'm doing it wrong!) - to have a native application on my computer for building the UX [macOS or Windows].

I'm not quite sure if I need a tool or a technique, but my current process is way too slow.