Acoustic posture is different from electric. The body is bigger, so your strumming arm rests differently, and the soundhole position changes where your hand should sit.
The casual position (most common)
Sit upright. The waist (narrow part) of the guitar rests on your right thigh (if right-handed). Neck angles up at roughly 30 degrees. Don't slump over the guitar; bring the guitar to you.Classical position
Right leg crossed under, guitar resting on left thigh, neck angled higher. Used for fingerstyle. We'll come back to this when we start fingerpicking in lesson 8.Strumming hand position
Your right elbow rests on the upper bout (the curved top edge of the body, just past the soundhole). Your hand floats over the soundhole, not in it. The wrist drives the strum; the elbow stays loose but anchored.Try it for 2 minutes
Sit with the guitar in casual position. Strum across all six open strings, just down-strokes, slowly. Watch yourself in a mirror or use your phone camera: are your shoulders relaxed? Is your strumming hand floating over the soundhole, not slamming into it?Bad posture compounds. Five minutes of practice on posture now saves you a year of "my back hurts when I play."
Next: your first chord. The open G.