A lot of slumping and over-arching results from holding the pelvis at the wrong angle. So some slumping and over-arching can be corrected quite simply by changing your posture.
Again, it’s best to have an experienced meditation teacher check your posture, but if you can’t manage that then there are some self-tests you can do.
Slip your hand under your buttocks (I bet you never thought meditation was going to be so much fun!). There are two little bones that protrude downwards through your buttocks. We meditation teachers have a technical term for them: we call them the “sitting bones”. There’s probably a more anatomically correct term, but we get by calling them sitting bones.
If you can’t feel those bones then rock your pelvis back and forwards. Did I say this was going to be fun? You should feel the bones moving across the palms of your hands.
If the top of your pelvis tilts forwards (causing your back to over-arch), then the sitting bones slide off the back of your hands. If you tilt your pelvis backwards (causing the back to slump), then the sitting bones slide off the front of your hands.
When your pelvis is perfectly aligned, then the sitting bones point straight down into your hands (or your seat, once you’ve removed your hands!). This is what you want.
So, what you are aiming for is to have those sitting bones pointing straight down while your back is relaxed and at ease. This can be achieved when you have your seat at the right height for you, so play around with different heights of cushions or whatever, and see when your back is relaxed and your sitting bones are pointing straight down.
Another check is to repeat the exercise of tilting your pelvis back and forwards, but put your hands on the small of your back, noticing how it slumps when you tilt your pelvis back, and over-arches when you tilt your pelvis forward. When your sitting bones are pointing straight down, and when you have the right height and angle of seat, then the small of your back should have a gentle hollow, which is perfectly comfortable.