First, when sitting on the floor or ground (not in a chair) then it is helpful to have something under the tailbone to raise your backside up a few inches higher than where your legs rest.
I purchased a buckwheat zafu, which I like a lot. However, I’ve also had success with stacking a couple pillows to sit on, taking a couch cushion off the couch and sitting on that, and I’ve used a folded towel.
No matter which method you use, sit on the edge of it so that your tailbone is raised, but your thighs are not on it.
Second, I experimented with different legs positions instead of he normal cross-legged position. I find it more comfortable to sit with the legs stacked on top of one another. Right leg flat on the floor, left leg resting flat on top of it.
Third, now that the body is in a better position, it is easier for me to find my balance point. I straighten my spine by sitting tall and pulling shoulders back and down.
Then, I lean slightly back to find the point where my spine is balanced (since I’m usually leaning forward slightly up until this point) and stop at the spot where the weight is distributed straight down, so the muscles do not need to be as engaged to stay upright. Once there, take a deep breath, exhale, and relax the body and mind.
It is possible to find a balanced position in a chair too, but I haven’t practiced that as much. I have found that in a chair I can adjust the distance of my feet from the chair to change the angle of my hips/pelvis, which is helpful for finding the place of balance.
I hope this helps! Best wishes on finding what works for you!