Ideally you’ll want to use a high chair. If all you have is a regular height chair, you can place a firm cushion or enough pillows on the seat to make sure that your left knee is below the level of your left hip.
Your left thigh must slant downward so that your knee is below the level of your hip:
Do not lean forward while sitting:
Do not sit on any low, soft surfaces such as a couch – this would break the precaution of bending your left hip more than 90 degrees:
Keep your knees apart.
Do not cross your legs:
Don’t rotate your left leg in, as shown in reaching for phone while sitting: