Go to the main page for your unit offering and choose to submit the unit guide for approval. Your Head of Department (or someone they've delegated) will review the Unit Guide. If the Unit Guide meets the standards required, they will approve the Unit Guide – then it will come back to you to publish. If they find any problems during the review, they will "decline" the Unit Guide – it will go back to Draft mode but you'll be able to see the reason why it was declined. If you've joined several unit offerings into one, then they are approved or declined together, not separately.
Further information can be found in the Approving and Publishing a Unit Guide.