The point of this exercise is to focus on pectoral isolation as much as possible without letting the anterior deltoid and triceps take over the movement.
While laying face up on the bench, holding the weight in each hand, keep the palms of the hands facing each other. The elbows should be straight out to the side of the body with the upper arm (shoulder to elbow) parallel (or lower) to the floor, while keeping the forearm perpendicular to the floor. Once in this position before moving the weight, make sure to keep the shoulder blades retracted (pull down and back into the bench, essentially squeezing the bench with your upper back, and initiate the muscle by squeezing the muscle being targeted (chest.) While pressing the dumbbells up, try to keep the forearm as close to perpendicular to the floor as possible. This ensures proper tension being placed on the working muscle (chest) and not the triceps and anterior deltoid, and will eliminate joint and elbow pain/discomfort. Think of drawing the elbows together instead of the hands, not oving up and down, but moving up and n and out and dow laterally, closing and opening the chest or shortening and lengthening the muscle.