UI/UX designer for a web application

I need a UI/UX designer for project that is already published. The project contains 2 websites and 1 mobile app.
I am in search of a person that can get attached for a long term and design changes in screens as per the user stories provided to him/her.
The user stories will be provided whenever new tasks come and he/she will have to develop the changes in the screens so that the developer can develop them.
The payment will be done according to the number of user stories provided every time.