The decline machine press predominantly targets the lower portion of the chest, however in general any chest excercise will target the entire chest area.
To start, adjust the seat so that the bar where you place your hands is in line with the center of your chest. Your shoulders should be pinned back, and pressed downwards. You can choose between a wide grip and elbows flared out - which will target the chest more/triceps less, or a narrow grip and elbows tucked in more which will target the triceps more and chest less. Once ready, press upwards and hold when fully extend, slowly coming back to the starting position.