The aspect that perhaps makes trousers the hardest to fit on yourself, is how to handle ease in the crotch seam.
This seam is suspended at either end from the waistband/waistline, rather like a catenary:
A catenary is a curve that an idealized hanging chain assumes under its own weight when supported only at its ends in a uniform gravitational field.
But the forces acting on the crotch seam line are not uniform.