How is this confusing?
You can purchase L1 leaders with ONP and sometimes hardware. A few are awarded during certain single player campaigns.
Upgrade your Leader Academy and research the different Security Teams. This will let you train them to higher levels in the Leader Barracks.
If you purchase two L1 leaders (as example: Brooks from Alpha Team), store both in your ware house and train them to one Brooks L2 leader.
If you purchase another two Brooks L1 leaders, store both in your ware house and train them to one Brooks L2 leader. Now you have two Brooks L2 leaders.
These two Brooks L2 leaders can be trained to L3.
Rinse, repeat for L4.