Both within science and society, transdisciplinary approaches are increasingly employed to address today's sustainability challenges. Often transdisciplinary research processes are structured in three core phases: a) problem identification and formation of a common research object; b) co-production of solution-oriented and transferable knowledge; c) embedding co-produced knowledge through transdisciplinary reintegration. In all phases of this ideal-typical model, the involvement of non-academic actors is essential to meet the challenges of real-world problems, and of transformative research practices. Despite existing guidance for the core transdisciplinary process, its initiation often remains an uncharted area because of its strong context dependency. Based on a concrete transdisciplinary case study addressing sustainability transformation in Transylvania, we bring together our learned experience with initiating a transdisciplinary process using a research-driven approach. To this end, we introduce the notion of Phase 0, as an initiating phase prior to beginning an ideal-typical transdisciplinary process. Within Phase 0, we propose three empirically and literature informed sub-phases: Sub-Phase 0.1) selecting the case study; Sub-Phase 0.2) understanding the case study context from a transdisciplinary perspective; Sub-Phase 0.3) fostering premises for coming together. We outline the general rationale behind these sub-phases, and we illustrate how we carried out each sub-phase in practice. By deriving cross-cutting lessons from the three sub-phases, we enhance the practice of transdisciplinary sustainability research with the aim to leverage its transformative potential.