I believe that there is a major distinction between an idea and a concept. Ideas only exist in our mind but a concept have to be manifested in order to be able to share them with others. 
Everybody has hundreds of ideas, but without discovering it’s hidden variables those are not getting conceptualised and remaining ideas only. I found it very important to specify a concept of any project as detailed as possible through research. 

What problem is that we are intending to solve by the product? Who is the audience? How will the product be able to help them? And so on..

These questions must be answered.

When the project has got well documented, I often sketch several variations of the interface layout. Okay. Once it is done..

I craft some wireframes and a clickable prototype.

No need to be pixel perfect… Rough, rapid, solution oriented.

If it turns out that our concept is invalid in some points, further design iterations can be more cost efficient than making changes in development.

The concept seems to be solid. The prototype is done. Now we should make an observation on how our potential end-users will use the product.

We have to do user tests on our prototype. It has to be well organised, and recorded for further assessments.

So, if they haven’t got some important parts.. Redesign those and do the validation again.