Typical liquid rocket stages stack the fuel and oxidizer vertically in separate tanks. If all you need is the center of gravity and don't need to be extremely precise, you can model this as three point masses: one for the dry structure, one for the center of the fuel volume, and one for the center of the oxidizer volume. As fuel is consumed, the fuel mass decreases towards zero and the fuel volume's center of mass moves downward from the center of the fuel tank to the bottom of the fuel tank; likewise for the oxidizer.
That model in effect treats the propellant tanks as cylinders. Real propellant tanks are more often ellipsoid-ended capsules than simple cylinders, so for a better estimate you need to break down the tanks into multiple sections.
Solid stages or boosters generally burn from the inside out along their entire length, so the fuel grain center of mass doesn't move as much (though the grain cross-section likely varies along the length of the booster, so it does shift somewhat). For rough purposes you can model this as a fixed point mass for the casing and a non-moving, but reducing, point mass for the grain.