The comment above still fits, You can obtain mass of the earth by doing the simple pendulum method to obtain g from the equation √g = 2π√L/T

g = earth's gravity L = length of the pendulum T = time period

Note that simple pendulum give adequate answers for small angles only, if you swing the pendulum to wide you won't br arriving at the earth's gravity.

then equate it with g = GM/r²

It's actually very difficult, because we have to derive it from gravity, and measuring gravity is very difficult. Actually, measuring the acceleration due to gravity is quite easy, that's what /u/lacgibra described. The trouble is measuring the gravitational constant. Of all the fundamental physical constants, the gravitational constant is the one that is known to the least accuracy. The classic experiment for this is Cavendish's experiment, and indeed this was originally conducted in order to estimate the mass of the Earth. This is still basically the technique used, but with much more precise equipment.

Fun fact: We know the mass of the other planets as a ratio to the mass of the Earth much more precisely than we know their absolute mass. This is because the uncertainty in the gravitational constant is greater than the uncertainty in the acceleration due to the planets that we can measure.