Before I start, there is no 'official' method of holding a paintball gun. However, my way is the best way :)
The purpose of holding your marker this certain way serves three purposes: profile, ergonomics, and ease of use. You want to find the best way to hold your specific marker that is comfortable and easy to aim with, that allows for the lowest profile possible.
Keep the tank in your shoulder. Some people prefer their tank above their shoulder, however this makes it especially difficult to pivot the marker when breaking out. Try the two and figure out what is best for you.
Once the tank is comfortable in your shoulder, bring the back of the marker so it is in line with your nose. If it is difficult to aim, slightly tilt the gun until you can see clearly past the feedneck. It is important the back is in line with your nose so that wherever you look, your marker aims.
When holding the marker, don't stick out your trigger hand. This might make it easier to walk the trigger, but makes you a much bigger target. Also, with only one hand on the gun, a player is more susceptible to shaking the gun. Even the slightest movement can throw off a shot.
Keep your elbows tucked in. Sticking them out provides a large target.
Hold the marker firmly and push it slightly against your shoulder. A loose grip allows the gun to move with each shot, affecting the overall accuracy of your shots. This is very important when running and shooting at the same time.
If you have a large milsim marker, it may be harder to keep a low profile. You may need to adapt this form in order to feel comfortable. Like previously mentioned, there is no specific posture, so feel free to hold your marker in whatever way is comfortable to you. Focus on keeping a low profile while still comfortably holding the marker.