At first it is a little confusing but you have to feel your way through. In a quick summary after a season game you unlock modifiers and they become available through the coaches you hire. You hire them for a set number of games, getting rid of the coach before his contract expires impacts your team negatively in regards to future coaches looking to sign with your team. If you keep a coach through their full contract you can dismiss them without penalty. When a coach leaves his modifiers go with him as well.
As far as applying the modifiers, each coach has a special area they specialize in which is represented by a letter (T, G etc). Each coach comes with 3 modifiers and when you highlight the coach you hired it will list what players on the team the coach's modifiers could be applied to. Basically you press on the modifier you want to apply and put it in the appropriate column of the player you want to impact. Once it is moved to a player you can move the modifier to another player on that list if you like. These modifiers will stay with the player for as long as you keep the coach.
Hope that helps!