Important players, relevant players and golfers
The first thing you must think about is the priority level of each player. You need to sort your players according to their skills and potential in "important", "relevant" and "golfers". You can't have more than three important players, since if you want to train Scoring and Playmaking, you will only have 3 training spots -and just 2 if you prefer Crossing-. Relevant players are those who might be valuable when sold in the market, and the golfers are simply those whose only function is to fill the empty places in your lineup. If you have more than a single important player, you ought to choose who is the most important one. Training - both primary and secondary - should be optimal for the most important player.