Firstly, we go on site and observe the processes and the situation in general.
If the issue is well known and the fix is pretty straightforward, we can make suggestions and set up an action plan (to be approved by yourself and the manufacturer), and then we can provide technical guidance on how to apply the plan. We report progress to our client.
If the issue is relatively complex (e.g. several distinct problems to approach separately, or disagreement among the factory staff on what the issue really is), it usually makes sense to spend more time at the “plan” stage before going into the “do” and “check” stages. Getting the buy-in, not only on the issue but also on the action plan, from the people who will work on the solution is critical.