One of the first steering decisions to make on any ERP project includes whether your project approach is domain or process focused.
ERP projects are often 'domain' focused. This means they are structured around functional domains, such as finance, purchasing or inventory for example. The advantage of the domain-centric appraoch is that your domains often map directly onto modules in the ERP solution making this structure easier to manage with your solution provider.
An alternative is to structure your project around business processes such as order to cash, record to report, procure to pay etc. The advantage of the process approach is that processes usually cross functional domains so they provide a more complete end-to-end solution picture than traditional domain 'silos'.
The difference is that whereas the domains normally already exist as organizational departments, the processes may not and therefore you need to work to define the process set around which your project will be structured. I recommend a SAT approach.
Irrespective of whether your project is domain or process-centric, each domain or process must have a 'lead' in the delivery team who is the owner of the domain or process and responsible for its delivery.
Lesson: Make the domain or process decision right at the start of the project.