Interview your customers about a specific feature you want to implement. Ask them how are they already executing this specific activity today.
Then, sketch in a peace of paper the feature you think is worth building, and also draw some possible variations for the implementation.
Sketching example: A mobile app for event’s check-in.
Feel free to draw in real time, on top of your sketching, the feedback you are receiving for your customers as an incentive to them. Let them understand that is perfectly fine to critique your sketch with their most honest opinion, so you listen their suggestions and you actually can make the changes right away without any problem at all. That is why paper sketchings are much better than higher quality mock-up tools with fancy graphic design.
“The lower the quality of the mock-up, the more the customers feel comfortable to invalidate your assumptions.”
This is the result of the feedback we got from a customer on the app for event’s check-in: