One of the best pieces of feedback I got at Heroku was don't stress too much on it being perfect. In fact you can make it rough as if drawn on a napkin so it intentionally isn't visually polished and it accomplishes its goal. I in fact started doing some of this way back at Heroku 9 years ago and the approach worked well. Now I've leveled that up with an iPad and paper.

Yes, there are places that have designers and they're a great resource, but don't let that be a blocker. If a company has a designer that can be dedicated and support such awesome. If not just try something and see what works, then keep using it.

Edit: An example of very crude/napkin style drawing - http://www.craigkerstiens.com/2011/11/07/how-heroku-works-ma...