The first step is a concrete written application for full file access with securing of relevant materials.
Under section 51 StPO the accused and their counsel have the right to inspect the file of the investigation and to make copies. This right exists in principle from the start of the investigation. It is a central instrument of defence to understand the evidence and to respond in a targeted way.
What to do now: First, submit a written application for full file access to the prosecution or the competent court, citing section 51 StPO. Second, after receiving access, secure all relevant materials and systematically evaluate them. Third, keep the deadlines for the detention hearing in view and insist on file access in time before the hearing date.