for example, take a disc of mass M and radius R.
take a small circular portiopn on the disc of width dx at a distance x from the centre.
mass per unit area of the disc is constant through out.
the mass of the is part=(M/pi R2)area of the small part
=(M/pi R2)2 pi x dx
moment of inertia of this part=((M/pi R2)2 pi x dx)x2
moment of inertia of the disc=?(M/pi R2)2 pi x3 dx
put limit from 0 to R