What are the steps to take to ensure a great UI/UX is made.
Go onto https://dribbble.com and scour the place for quality design work. Once you've found some great pieces, look at what else those designers have done.
If their portfolio fits well with your requirements, contact those designers and tell them about your project.
Avoid sites like 99designs.com and oDesk in my opinion.
I would build on top of Danny Halarewich's answer.
Having gone through a sorrow experience with Dribbble, I would recommend you also check that the work of the designers you contact is live, so that you can require them to send some references.
Reach out to those references before deciding on a designer.