If you are coming from a graphic design background, this part can be pretty intuitive. When you are done building and designing refer back to the section on validating your designs with users.
How I Taught Myself UX Design
Sarah Croughwell

The biggest issue graphic designers have when making the switch to experience design is prioritizing the user (and user needs) over aesthetic considerations.

Many graphic designers want to make beautiful prototypes, not realizing that — until a users can be observed interacting with a design — the design is just a guess. Even if you base your designs on a ton of research, you won’t know how effective it is until you observe real people using it.

I hate the term “validating your designs” it implies your designs are good, and you simply need to validate that assumption. It also implies that it’s a task, opposed to an ongoing process of optimization. Testing doesn’t validate a design, it can uncover opportunities for optimization.