Offshore Wind Power Plants (OWPP) are essential to achieving Europe’s decarbonisation and energy security objectives. As new projects are developed farther from onshore grid connection points and at larger scales, the complexity of their transmission systems continues to grow. In several markets, developers are responsible for the engineering, construction, and sometimes operation of the offshore transmission grids, making early-stage design decisions critical to project costs, risks, and business-case viability. This paper presents a methodology and sets of best practices for the early-stage design of High-Voltage Alternating Current (HVAC) transmission systems for large OWPPs. The proposed approach supports developers in progressing from limited preliminary information toward technically robust and commercially viable transmission concepts aligned with project constraints, state-of-the-art technologies, grid code requirements, and regulatory frameworks. Key techno-economic decisions are addressed, including the selection of transmission system topology, reactive power compensation strategy, harmonic compliance requirements, and identification of essential input data, modelling assumptions, and grid studies. The paper also examines cable active power transfer capability and loading limits as well as the impact of regulatory frameworks on the design decisions. The methodology is practiced on two study cases from Vattenfall’s offshore portfolio and supported by PowerFactory-based grid modelling and simulation studies.