Once a therapist has an idea of what is troubling a client, they will both work together to develop a treatment plan. In my opinion, it is generally pointless for a therapist to develop a plan without the input of the client - to do so assumes the therapist "knows best," and most people don't have a positive experience in therapy if this is the case. Ideally, the client and therapist work together as experts - the client as the expert on himself, and the therapist as the expert on mental health.
Unlike diagnoses, there is no set, formulaic way to create a treatment plan. A treatment plan could be as simple as "Client will attend therapy until they report less anxiety," or it could be very long and only contain objectively measurable criteria, such as "Within six weeks the client will experience less stress as evidenced by a score of less than 65 on the Outpatient Questionnaire." In my opinion, it is a good idea to have some idea of the goal of therapy, but having a complex treatment plan is often impossible to follow and can begin to be more important than the actual therapy. For the treatment plans on this website, I'll follow a fairly basic formula, but one that is actually more complicated than I would personally use in real life. The basic idea is "what does the client want? What will be done to help the client with their complaint? How can you tell when therapy is coming to a close?"
Each treatment plan will be different, in no small part because each person is different. Additionally, each therapist has their own style and approach. The interventions mentioned on this website are not "right," and are often not the interventions I would personally use. That said, some interventions do work better with certain problems than others, and it would be foolhardy to ignore research into effective treatment.
As mentioned previously, ideally a treatment plan is created by both the therapist and the client. Since I do not have access to any of the characters I will simply infer what they would likely desire from therapy.