Collaborative industrial robot (cobot) systems are deployed to automate tasks or as a tool for Human-Robot Interaction (HRI) scenarios, especially for manufacturing applications. A large number of manufacturers of this technology have entered the cobot market in recent years. Manufacturers intend to offer easy control possibilities to make cobots suitable for different user groups, but there are few evaluation tools for assessing user interface (UI) design specifically for cobots. Therefore, we propose a set of design guidelines for cobots based on existing literature on heuristics and cobot UI design. The guidelines were further developed on the basis of modified heuristic evaluations by researchers with robotics expertise, as well as interviews with cobot UI/User Experience (UX) design experts. The resulting design guidelines are intended for identification of usability problems during heuristic evaluation of the UI design of cobot systems.