1. Click on any white area of your canvas, the properties section on the right side should have title: Properties (BPMN-Diagram)
2. Open the properties section
3. Expand if needed the "Extra" section of your properties
4. Find the "Variable Definitions" property and click on the arrow at the end of the input box - this will open the variable definition editor
https://community.jboss.org/servlet/JiveServlet/downloadImage/2-778714-19938/450-248/sc2.png
5. Click on "Add Variable":
https://community.jboss.org/servlet/JiveServlet/downloadImage/2-778714-19939/450-236/sc1.png
6. Enter your process variable name, and select either a standard type (optional) or a Custom type (optional). If you don't select anything it will default to Object type.
7. Add more if process vars if needed.
HTH
