Just because customers care about the problem, it doesn't mean they like your solution.
And as innovators you will be too biased in favour of your solution. Especially if you are not going to buy and use it yourself.
When you have a specific idea about your solution, how it will work and what it will look like, it is time to meet real people - potential customers - to get their response.
Right after step 4: Prototype, when you have some kind of visualization of your product.
To do a solution test you will have to show potential customers what your solution is. Not as a working and well designed product, but as something visual that will make them understand what it is in images. Not in words.
Preparation:
Build a prototype, model or visualization. Quick and dirty will do for now and often invites more feedback.
Identify and contact potential customers or the closest you can get.
Presentation:
Explain the problem as you see it – and as customers in your problem test saw it.
Show your solution – prototype, model, drawing or other visualizations.
Go through the details.
Explain what is special and how you are different.
Learn from customer reactions. Look and listen - also between the lines.
Steve Blank & Bob Dorf: The Startup Owners Manual. 2012. The method is well described in chapter 6, p. 227-256.