The development phase is a journey of discovery. You carry out planned experiments. You learn from setbacks, and try out different tracks. You involve external expertise that proves necessary. You document results, and gather sufficient evidence that the newly developed practice works. You ensure that all questions that had been raised in the planning phase are answered.
Sometimes a track comes to a dead end, or you need more space than you have been given for this development phase. Then you go back to the inspiration phase and take a new run-up to the planning phase.