Your eyes should be level with your screen and be directly in front of you to achieve this, you may need a monitor stand f the screen is too high or too low, you'll have to bend your neck, which can be uncomfortable and cause long term pain.
Your wrists must be in line with your forearms and elbows should be by the side of your body so your arm forms an L-shape at the elbow joint.
Ensure your chair is adjusted to fully support your lower back , the back of the chair should be completely vertical. A correctly adjusted chair will reduce the strain on your back. Get one that is easily adjustable so you can change the height, back position and tilt.
Place your feet flat on the floor. If they're not ensure you have a footrest/stool, which lets you rest your feet at a level that's comfortable. don't cross your legs, as this may contribute to posture related problems.
Take regular breaks and ensure you are using your sit stand desk to its full potential. Don't sit in the same position for too long. Make sure you change your posture as often as is practicable. Frequent short breaks are better for your back than fewer long ones. It gives the muscles a chance to relax while others take the strain.