Hey, I am new here and I have some experience of singing. I practice in the car since that's the only place I can comfortably practice without being anxious. Will I still be able to follow this guide if I have to sit in the car? I'm still in school and I usually practice during the break where I can stand but I feel rushed because I have a limited time to practice where as If I'm in the car I can take my time but I feel like I am hurting my voice when I sit down. 

You can practice seated, but it sounds like standing is a better option (since you mentioned feeling pain while seated)! I encourage you to look for another private setting (the garage, bathroom, closet, etc) where you'll feel comfortable and can practice standing.