Let us quickly understand why we need a meditation cushion in the first place, which will help illustrate how you should sit on a meditation cushion. There are some wrong ways to use such a cushion but there is no one right way. It depends on your skeletal form. While we are all humans, we don’t have the exact same skeletal shape. It is also not about ethnicity or genetics entirely. It is more about how someone’s spine and rest of the skeletal form respond or act in definite circumstances.
A meditation cushion is necessary because human spines have different degrees of curvature. The shin bones, the hip and the knees form different kinds of alignments depending on the skeletal structure. There are three possibilities with the shin bones, hip and knees. Some people can easily sit on the ground or a flat surface with their hip bones in alignment with the knees, both at the same level. Some are more comfortable with their shin bones and hip being in perfect alignment and the knee joints actually at a lower level than the hip bones. These are easier postures to maintain.
The problem lies in postures where the knees are raised and not at a level with the hip. It happens to most people wherein the knees are elevated on either side when one sits down crossing and folding the legs at the knees. For these people, a meditation cushion is imperative. Now, you can sit on a meditation cushion, which will give your hip the height you require and your knees will be at a lower level on the ground. This posture will be more comfortable, especially when you are expected to sit for a really long time.
If you don’t need so much height or you don’t like the elevated positioning of your hip in alignment with the knees at a lower level, then you can use the meditation cushion under your thighs. This is particularly helpful for those who want a bit of an elevated position but not one where the knees are substantially elevated relative to the shin or hip bones.
Many people use meditation cushions just for the comfort. Sitting on a mat or on the ground, which is a hard surface, may not be very comfortable after a few minutes. You can always tweak how you use such a cushion as long as your physical posture is upright and your spine can balance its weight by the virtue of its curvature.