Typically, the parties themselves are able to determine parental rights. Folks who agree to a parenting plan and do it in an uncontested or non-adversarial way do a better job of buying into that agreement and it often goes more smoothly than a plan that is ordered by a judge. If the parties are unable to agree, then one of the judges makes the decision about what the arrangement would be after each party has a chance to be heard.
Child custody should be established in court. Oftentimes, people reach agreements outside of court and the problems they run into is that those agreements are not enforceable with the court. Unless a judge signs such a plan and makes it an order of the court, it is unenforceable. It is a good idea for people to go through the legal process of having one of these plans done.
When handling the child custody component of their divorce, the best thing a parent could do is craft a case with their attorney that considers each factor by itself and prepare and present the best case that that parent is the most suitable primary parent or custodian under the factors of TCA §36-6-106.