Method in which lock-in detectors process signals from standard ac susceptometers, particularly in the detection of higher harmonics, is analyzed in details. The exact formulas have been derived and checked experimentally by measurements on soft ferromagnetic sample, using several available lock-in amplifiers. The reasons why the proper phase adjustment has to be implemented in the protocol of higher harmonics measurements have been elaborated. The procedure of the lock-in phase adjustment is described, enabling separation of Fourier or Taylor components of hysteretic ac susceptibility into real and imaginary sectors.