The statutory instrument for schools that has the most detail is the Computing Programme of Study. This is a concise but flexibly written document, so it gives a clear steer to the content. The first thing to do is to relate this content to the schemes of learning and lesson plans the school already provides. There are many resources to support this in the CAS resources repository but quite a lot of what the school already does is likely to still be usable. There are free lesson plans licensed for sharing that can be modified to suit any individual needs and many several are also associated with a range of baseline tests (see Further Guidance for references). This wealth of resources should mean that no-one is in the position of having to start without any resources at all.
Planning your Curriculum & Getting it Right
Embedding computing within the school curricula is a journey that will require the support and collaboration of the entire staff body. A good action plan, leaving sufficient time for planning and having reasonable expectations of yourself, your team and your learners will help to ensure success.