How do you recommend to start a sale process on a creative industry where you have to show creativity and seriousness in the same time ?

First of all, all sales processes are not same, based on the product/material you want to sell on the market  and according to that type of product/material have to develop a sales process. In fact, as per your question  unable to recommend sales process unless you have to identify the product/material.

To show the Creativity, you have understand more and practice more on creative thinking method and eventually you can develop creativity better then what you have now.Seriousness is type of behavior and it is different from person to person, there is no any short cut method to show the seriousness otherwise the person has to have that capacity .  

Product Knowledge. You must know the features of the product that you are selling.