how to charge for design work

i have a client who wants a design job for a product launch> This is a new client for me and a new line of work. I have a freelancer who will do the work, but how do I charge.

it is easy if the client likes the work, but do you charge for your time if they don't like the work or at least they say they don't?

How do you ask for them to pay for something they are not even certain they are going to like.

I hope this makes sense.