Nonlinear effects in black hole perturbation theory may be important for describing a black hole ringdown, as suggested by recent works. The influence of these nonlinear effects on quasi-normal modes of black holes is still poorly understood. I will show that a generalization of WKB method, the so-called technique of uniform approximations, can be used to accurately compute 1) nonlinear amplitudes at large distances in terms of the linear ones, 2) linear (and nonlinear) quasi-normal mode frequencies, 3) the wavefunction for both linear and nonlinear modes. This technique could be used to improve ringdown models by adding nonlinear features without introducing any supplementary free parameter for data analysis purposes.