1) Install Ribbon Workbench from here
2) Import solution in your Dynamics CRM 365 instance
3) A button "RIBBON WORKBENCH 2016" is created in Solution Page. Click on it
4) Now new window is opened. Select your desired solution containing entity to be customized. It is better to create a separate solution to be used for ribbon customization and contains only desired entity. In this scenario I have created a new solution "Ribbon Solution" contain only Case entity. Here I am selecting solution Ribbon Solution
5) Now solution is opened in ribbon workbench. Select your desired entity and on button right mouse click. Now select Customize Command. (In my case as button is already customized therefore this command is shown disabled)
6) Now in command section, select the generated command and click on Add Enable Rule. Now select Add New Enable Rule.
7) Now click on Add Step & select CustomRule. There are others rules to apply enable rules on button. Here I am only showing the CustomRule.
