How do you progress trough designing music for somebody else ?

Let me be clear on this. I will never ever complete an assignment at once. I believe it is much version to let the client listen to several test versions before I "waste" all my time doing something which turns out to be not good. I start by throwing together a basic arrangement that will be the base of what the piece will sound like. Then I send it over. At that point you can decide what stuff needs to change (if any).

Gradually (and on a daily basis if needed) I can send examples over to see if it suits the client's needs. Too often it happens that musicians are required to build music but which ends up not being the taste of the client and then they have to restart from scratch which is a waste of creative potential and effort.

This symbiosis will ensure that the musician remains on track while the client can steer the creative design without wasting eachother's time.