For a nebula we construct a model, which is a spherical shell defined by inner and outer radii, density distribution and velocity field. The Teff and the luminosity of the central star are assumed to agree with observations. Then the photoionization structure is calculated and the emission lines are integrated. Having found the density from analysis of images the deduction of velocity field in order to fit the observed shapes of spectral lines is relatively easy. A detailed description of computer codes applied for the analysis of nebular spectra is presented in our earlier paper: Gesicki et al. (1996). See also Górny et al. in this volume.