The present tutorial provides an overview of the time-dependent configuration interaction singles scheme applied to nonlinear ionization over a broad photon-energy range. The efficient propagation of the wave function and the calculation of photoelectron spectra within this approach are described and demonstrated in various applications. Above-threshold ionization of argon and xenon in the extreme ultraviolet energy range is investigated as an example. A particular focus is put on the xenon 4d giant dipole resonance and the information that nonlinear ionization can provide about resonance substructure. Furthermore, above-threshold ionization is studied in the x-ray regime and the intensity regime, at which multiphoton ionization starts to play a role at hard x-ray photon energies, is identified
