Camry Hybrid regenerative braking
 

How do you use the manual shift mode to maximize regenerative braking and thus improving gas milage.

It doesn't.
It is like the B mode on the Prius. That 'gear' tells the car to let the engine spin up for braking. Shifting the 'gears' on the Camry hybrid just gives you different degrees of that engine braking instead of just one setting.

It will be useful on steep hills, but may actually result in less regen braking being applied. Of course, going do a hill can quickly fill the battery, and the system will spin up the engine for braking on its own.

The best way to maximize regen braking in a hybrid is to just use the brake pedal. Even then, the small size of the battery the limit on how much energy can be recaptured while braking.