I think you need to go old school and find a goat to squeeze between your knees , goat probably won't be too thrilled about it though.
Tilt your pelvis forward and up , but not to the point where your quadriceps muscles tense up , back it off until the thighs relax and the glutes are just slightly firm.
Pull your hands back until your relaxed fists are in line with your chest , push your chest out slightly to open up the shoulders.
Visualise a piece of string vertically pulling up the top of your head , this will lengthen and straighten the spine , I think they call it "sing" if I remember correctly.
Back of shoulders to knees forms straight line called the "Wing Chun slant body" but you don't have to worry about it , what I have already told you should theoretically enable you to produce good posture if you follow it properly.
I am TST lineage.