I would say that the next step would be to document a high level summary/description for each use case.  This would be no more than 2 or 3 sentences and it would be something like this. "This use case describes the process by which <actor> interacts with the system in order to <perform something of value>..."

Review the use case diagrams and the descriptions with the stakeholders to ensure you have not missed anything at the high level.

The next step would be to detail out each use case by:

• Identify the Use Case pre-condition

• Determine the desired end-goal aka your post-condition (what should ideally be true once the use case ends)

• Documenting the main flow (happy path)

• Documenting Alternate Flows

• Documenting Exception Flows

Review the entire use case specification with the stakeholders.

Here are some templates and cheat sheets related to use case specifications.

Hope this helps!

Adrian