Pattern libraries are the most beneficial when your organization has a large range of products that use similar functionalities and features.
Here is a step by step approach to creating your own pattern libraries:
1. Identify features, functionalities, basically patterns that are repeatedly used across the products. Examples include Sign In, Sign Up, Form Designs, Notifications, Error Messages, Emails, Content Types etc.
2. Research various use cases for each of these patterns, then Design patterns and Document the behavior details for your organization to uptake.
For example the Sign In page may have a few use cases across your products. There could be instances with 2 fields for Sign In and in some instances 3-4 fields. Likewise there could be use cases and variations of other design patterns across your product.
Include behavior documentation such as hover, animations, on clicks, keyboard actions etc related to the patterns that can't be explained via designs.
3. Include notes about flexibility, use cases and the visual style of these patterns. Add Contact details for any questions.
4. Keep patterns updated & fresh since the Interaction Design keeps evolving in this fast paced technological world. Keeping patterns up to date is more critical than adding new patterns.
5. Each pattern design takes much more time than creating a single design. So, ensure quality and detailing. Understand that these patterns are to be uptaken not only by other designers but also by developers and project managers.
Finally - Continue growing your pattern library. It is a long term investment eventually resulting in consistency and shorter design time across your products.
All the best. Share any more knowledge that you may have.