hello guys, do you think, when I want to seduce a girl, I should be relaxed (without worrying too much of my body) or is it better if I apply the advice on body language (ie back straight, legs apart ...)?

Be relaxed. AFTER the set some time, go back in your mind and analyze your body language, tonality, eye contact, whatever you want. But while in a set, just stay in the flow of the conversation. One of the things you'll see from some of the gamey individuals out there is that you can just TELL that they're trying to follow some playbook for the exact "body rocking" or hand gestures and it looks robotic and it is easy to see that they are not in the moment but rather thinking of these technicalities when in reality, a light and bantery flow is the most important aspect.

Relaxed translates to a 'relaxed' posture. I'd work more on being comfortable with yourself than faking some positions which in the end may just put off an awkward vibe.