Once you've chosen a plan, it's mostly set-it-and-forget-it - the player will grow in the background throughout the year.
One way of knowing what plan is best is by looking at the ETA in the top-left corner that shows when the player's overall ranking will go up. Keeping your players on Balanced is the easy way out. By default, each player is set to 'Balanced,' which spreads the points evenly across the most traits without focusing heavily on one area. Alternatively, the 'Defensive Wide Back' option raises his aggression, standing tackle and reactions, but not the other traits. For example, if I want Alfonso Davies to be a wide full back, I would choose the 'Attacking wide back' option and his vision, crossing, curve and short passing abilities would increase more than others.