I posted a response in the public sector section of this forum, that may be relevant. Although you ask about where to start, one of the challenges I've seen is where to end. Here's a possible framework:
1) Supply Chain Policy Manual. This should include the key policy issues for adoption in your Department and for all buying Depts. Keep this very legalistic and concise. The audience for this document is all levels and sections who procure goods, assets or services or get involved in the disposal of assets, so also keep it easy to read and distribute.
2) Delegation of Authorities. This must specify the different layers in the organisation and the spending levels that they are authorised to approve. This should also specify the different committee structures that must be followed for different spending levels or commodities. As committee membership and delegations change frequently, keep this seperate from the policy manual.
3) Commodity Sourcing Strategies. This is the sourcing strategy and or approach that is used for different commodity types. For example, Consultants (one of the commodities) or vehicles or ....products.
4) Supply Chain Process Manual (or Blueprint). This manual is probably optional if you are in a relatively small organisation but it is critical if you have a large organisation or multiple offices across the country. Include the actual process flows that should be followed in each of the areas of the SCM Framework. Also use this manual for your current templates and forms. Some Departments are looking at making this available on an intranet site. This is where you include the systems flows etc.
Hope this helps.
Please advise on the overall structure you eventually use.