This can be a club team, school team or select team. The principles are the same for each and may vary slightly. This needs to be done before you see your players.
Some coaches like to select their players and then think of the systems that they are going to use.
Knowing what you want and how you will come to your conclusions must be conveyed to your players. In fact this is all part of Athletes Rights. Players must know what is required of them, how you will select. They then must be notified of the exact reasons why they were let go. When it comes to national teams or provincial teams they need to be told clearly and then if they wish they are to be given an appeal process.