I've had a lot of luck throwing something together in Bootstrap or similar, and then when a designer gets involved with the project, they approach it as more of

I've had a lot of luck throwing something together in Bootstrap or similar, and then when a designer gets involved with the project, they approach it as more of a redesign: they can get familiar with the app, understand what it's trying to do, and then go from there. Paper, in my experience, doesn't adequately communicate some things (for instance, more complicated UI interactions are somewhat hard to model in a reasonable way on paper).