By this stage of the process you will have already spent a considerable amount of time communicating with colleagues, gathering information and coming to a decision about the best course of action. Before you can move forward with the approach you have selected, you may need additional resource to carry out the work. Even where resource is not required, having buy-in from senior colleagues will be important to ensure that they understand what you are doing and why.
The DPC’s Digital Preservation Business Case Toolkit provides a template and step-by-step guide for building a business case. Some of the preparatory work will already have been carried out (for example finding out about the content, assessing the risk and considering preservation approaches). Your main challenge will be in identifying what you and your organization need to achieve your aims, calculating costs and effectively communicating these needs.
In order to help establish methodology and costs you may find it helpful to carry out a small pilot project. This should help inform the business case and add weight to your arguments. A pilot project may be a substantial piece of work in itself and it may not be possible to do this without additional resource.