Grasp the barbell with your hands positioned a little more than shoulder width apart.
Unrack the weight and lower the bar down to your lower-chest line until it's an inch or two away from your chest. Then, raise the bar up again to the starting position and repeat the movement for the prescribed number of repetitions.
Perform this exercise in a smooth and fluid motion
Don't arch your back, keep it flat on the bench at all times
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, increase the weight.
This is a classic exercise for the lower chest area. However, it's important to point out that this exercise is something you should be doing only after you build a solid (muscle) foundation with regular flat bench and incline presses. The decline bench isn't something that should be at the core of your chest exercise routine. In other words, it's not an exercise that you build an entire chest workout routine around.
Having said the above, this exercise is great for your lower chest area and will really form that line at the bottom of your chest that frames your chest muscles. Thus, it's a great movement to incorporate into your chest workout routine.
Last but not least, you don't really need to load up the barbell super heavy on this one. This exercise is really about muscle separation (creating the above-mentioned "line"). This means its a finesse movement where you will want to do more sets with a higher number of reps.