If you've completed the previous steps and still feel good about moving forward with your idea, it's time to get some things built!
But you will have to spend some money.
The goal here is to turn your napkin sketches into interactive designs that potential customers can explore.
You're not building something that "works" in a functional sense. You're building something that looks like it works.
You're building designs with predefined screens that users can navigate to by clicking on different design elements on your page.
And your focus is to highlight all of the core features that will need to be in play to really sell your idea.
If that's confusing, it's OK.
Because this is usually where you bring in a designer.
At this stage, I recommend you find a local freelancer. Not an agency. Agencies are pricey and at this point, it's usually not worth the money.
So find a local freelancer who has experience building interactive designs and ask them to show you some of their work.
If someone claims to specialize in user interfaces (UI) and user experience (UX), that's usually the type of person you're looking for.
Finding a local designer should be easy in most cases, especially if you're near a university and can hire a student.
But if you have trouble, you can find someone on Upwork.com but use that as a last resort.
Local is the better call here. You want someone you can sit down with.
Price-wise, I wouldn't recommend spending more than $5,000 at this phase and in many cases, it should cost you less. Although it depends on the complexity of your app.
Ultimately, if you believe in your idea, your potential customers believe in your idea, and your mentors believe in your idea, it's worth a shot.