Using your Drill Instructors:SI
Click the Training Center to access the Training Program.
You will see all production-line Units divided into Unit Sets.
In order to unlock different Unit attributes and branches of the Training Program, you must progress through each Unit Set sequentially.
To get started, use two of your Drill Instructors to unlock Training Phase 1 by clicking the "Training Phase I" button twice.
You'll notice that assigning each Drill Instructors takes time – this time will increase with each additional Instructor you assign to a Unit.
As you progress through the game and have successful missions, you will collect more Drill Instructors for your training regime.