Use a metronome. Set it to MM=120.
Take in air and "breath" (and sing silent inside) the first two bars with legato. Then the next two bars, etc. Take in air as quick and silent as possible.
Repeat this exercise but this time use a gentle "Ta" attack with the tip of the tongue. Keep the air going all the time like when doing it the first time legato. The tongue must not stop the air, just flip it lightly.
When you can do this easy try to "breathe" 4 bars. First legato, then with "Ta".