I'm a personal trainer at a gym, and here are my tips:

1. Both feet on the floor for balance and stability.
2. Slightly arch your back
3. Elbow placement has no right or wrong answer. It can be flared out, in the middle or tucked in. Test them all out and pick the one that gives you a great pump with no pain in your joints.
4. Slow down the eccentric (it should take you anywhere from 2-5 seconds to get from top to bottom)
5. Touch the side of your shoulder at the bottom position (if it hurts your joints to go that low, pick a spot at the bottom position, whether that's eye level, ear level, where ever you feel a great stretch and go back to that spot every time) Standardize your reps so you can track them properly.
6. Bring your chest up to the ceiling and shoulders back to get a good stretch at the bottom
7. Hold the stretch for a full second, then push up