It is time to code — and all the developers in the team roll their eyes and mutter "finally"!
A developer should choose one of the Behaviour Specifications they agreed to implement and do just enough coding to fulfil that specification and also implement the automated test for it. Simple – rinse and repeat with the next behaviour.
Whether or not a developer follows the Test-Driven Development (TDD) practice of Red-Green-Refactor is a personal choice. If a developer wants to, then they can implement the test automation code first. That approach has no effect on the practice of BDD, but depending on the skill of the developer, it can result in code that is more easily testable.
Developers also should remember not to write any automated tests other than those specified by the behaviours.