When designing apps for Apple Watch, these apps need to be designed differently from how you would go about designing apps for phone or tablets as they provide a different user experience. It is important to keep in mind and follow the Apple Watch interface guidelines documentation that Apple provides.
This document describes guidelines and principles that can help you design consistent user interfaces and experiences for your Watch apps as well as ensure that your applications are running efficiently within the Apple Watch platform. You need to consider the screen sizes for your custom layouts as well as the ease of use your app brings to the platform. Other areas are covered to ensure the consistency of your application while navigating from screen to screen; the principles of designing good user interfaces are also covered.
There is also information related to the proper use and appearance of views and controls for navigation, alerts, notifications...