There are many variations of lunges and different purposes for them in a program, I have to emphasize that form will vary with purpose and person. For example a forward lunge might be in a program to focus more on front leg deceleration whereas in a plyo lunge the athlete would typically be more balanced between the 2 legs.
With that said there are 3 positioning cues I want to see in 99% of sagittal (forward/back) plane lunges:
1) Front Knee, heel & toes in line with their hip. hips & Shoulders square, Back is long & Core braced
2) Back leg in line with hip, not flaring out and hip is at/near full hip extension (Glute max is active)
3) Slight forward lean with the torso: create a straight line from back knee through the hips, shoulders, & Ears
1 common mistake people make while learning a lunge is overarching their back to keep the torso vertical or "Chest up". chest the upper back strong which will keep your leading but the abs, erectors, obliques & glutes should all be activated, not sure your erectors.
See Lunge Variations for more specifics.
Below is a Strong vs compromised spinal postition