For example, let's say I have an idea for an app. Naturally I would think to design a few mockups, then post it on Twitter, Reddit, etc.
And then if people actually cared enough to sign up for an email list, then build.
So in this case, design -> sell -> build .
2. Meet people; ask what they do; wait for them to return the favor and ask what you do.
3. Pitch them on the problem you're solving — and how you are (intending to) solve it — by just talking them through it. Pretend it's something you've already built and that you're just describing the company you work for.
4. Gauge interest by who demands your business card.
Simple mockups can communicate an idea effectively enough to sell the idea. But there’s a large delta between a pitchable mock and a design that is ready to build.
Those early mocks rarely match the later designs in my experience, and I think can be kept under the “sell” umbrella.