A growing body of mathematics education research points to the importance of fostering students' mathematical creativity in undergraduate mathematics courses. However, there are not many research-based instructional practices that aim to accomplish this task. Our research group has been working to address this issue and created a formative assessment tool, the "Creativity-in-Progress Rubric (CPR) on Proving". This tool is developed to help students evaluate their own proving process, as well as provide instructors insight into students' self-evaluation and process. In this paper, we provide a brief description of the CPR on Proving and explain its implementation in two courses at different institutions. Additionally, we share several students' usages of the CPR and provide some practical implementation suggestions.