Start in a Support position, with your shoulders pushed down ( Shoulder depression ).
Slowly dip down by bending your elbows, descending with control.
Try to adapt the height of the bar or to place a support for your feet, to have something to step on once your reach the bottom position.
Form tips
How deep should you go? You dip will be considered complete by some people if your shoulders get at least as low as your elbows. There are benefits to going lower, such as increased stretch, leading to extra strength and flexibility gains, and facilitate the Rings muscle-up , but it will also increase the intensity of the exercise and possibly the risk of injury. You can progress to a deeper position over time as you get stronger in that position.
Risks
Eccentric training can quickly push your body to its limits; start with a very small volume (1-2 reps at a time) and progress cautiously.