In this paper, we propose a semi-automated approach for helping non-experts in BPMN to model business processes using this notation. We focus on a timed version of BPMN where tasks are associated with a range indicating the minimum and maximum duration needed to execute that task. In a first step, the user defines the tasks involved in the process and possibly gives a partial order between tasks. A first algorithm then generates an abstract graph, which serves as a simplified version of the process being specified. Given such an abstract graph, a second algorithm computes the minimum and maximum time for executing the whole graph. The user can rely on this information for refining the graph. For each version of the graph, these minimum/maximum execution times are computed. Once the user is satisfied with a specific abstract graph, we propose a third algorithm to synthesize the BPMN process from that graph.