You see lots of people lean aggressively forward when shooting a shotgun, back knee locked, front knee bent, or else they lean back, usually a habit formed by trying to handle too long and heavy a gun when they started shooting.
The proper stance for shooting a shotgun is fairly upright. Your knees are neither bent nor locked, just relaxed. Lean forward slightly, with just over half your weight on your front foot. The easiest way to remember is to think: “Nose over toes.” When you’re standing correctly, your nose should be directly above the toes of your front foot.
“Assume the position” in shotgun shooting means starting with proper foot placement and stance and then holding the gun in such a way that it can be brought quickly and smoothly to a firing position. The butt should be tucked very lightly under your arm. The barrel should be parallel to the ground or pointed slightly higher if there are bird dogs in the field. Be sure to keep the muzzle below your line of sight. Your grip should be relaxed, and your finger should not be on the trigger or even inside the trigger guard.
When hunting, whenever possible, I’ll take a short step toward the target and bring the gun from whatever carry position I’m using into this ready position while my eyes find the bird. Then I’m ready to make the shot.