Using a complex systems lens, we answer the following question: how can one engage in such a system to robustly benefit the long-term future? While existing research provides provisional answers on what could feature on the agenda of longtermist advocates, little progress has been made on how to design and implement agendas. We address this gap of translating research insights into direct engagement with policy processes. We provide an overview of what advocates must understand and build to have a positive impact, divided into two main sections. First, we review the literature on advocacy, lobbying and epistemic communities to identify best practices in policy engagement, notably including advice on how academics can constructively collaborate with policy actors. Second, we provide a framework, grounded in the literature, to guide longtermist advocacy: delineating considerations, guiding questions, and pieces of advice for effective engagement. We conclude with limitations and notes of caution on the use of the aforementioned framework.