Urgent action is needed to reduce greenhouse gas emissions and mitigate global warming, which leads to ecosystem degradation, extreme weather events, and economic and human risks. The Paris Agreement's goals can be achieved by establishing emission reduction targets and decarbonizing the economy. China, the largest emitter of greenhouse gases, aims to reach carbon peaking by 2030 and carbon neutrality by 2060. Urban areas account for 85% of China's emissions, making it crucial for cities to adopt decarbonization measures to fulfill the national targets. Therefore, many city governments have developed decarbonization plans to explore low-carbon development opportunities. Based on a client project, this policy report focuses on the Chinese context and aims to provide practical knowledge for urban development planners, policymakers, and the public interested in decarbonization issues. The report clarifies the concept of decarbonization planning and provides a comprehensive planning guideline, including greenhouse gas inventory, future emissions forecast, setting decarbonization targets, and developing action plans specific to the region. To illustrate the practical application of the proposed guidelines, the report also includes a case study of an island city in northern China. The GHG inventory shows that the study area's emissions decreased by approximately 56% from 2016 to 2021, primarily due to the promotion of electrification in various industries. The LEAP model-based emissions forecast for the next 40 years reveals the feasibility of the island city achieving carbon neutrality by 2035 under a stringent low-carbon scenario. Based on this analysis, we design and compile a decarbonization action plan for the island city, presenting information on guiding ideology, principles, cross-cutting strategies, necessary decarbonization initiatives for specific sectors, and capacity-building guarantee systems.