The STELLIM interferometer aims at providing fast and reliable imaging of the surface of cool evolved stars by combining the light of 13 telescopes in the visible. It is designed to take advantage of the existing VLTI infrastructure. Optical fibers carry the light from the telescopes to the VLTI delay line tunnels where the geometrical optical path lengths are compensated. Using a mirror switchyard, several possible combinations of 7 telescopes are selected in turn and their light is recombined on a dedicated table in the VLTI laboratory. One single night of observations is enough to reconstruct a reliable interferometric snapshot image. STELLIM aims at producing stellar surface images at a high temporal cadence. It will deliver a complete spatio-temporal characterization of surface and circumstellar environment structures of bright evolved stars, which is pivotal to understanding the way those stars lose their mass in the interstellar environment and their evolution into supernovae. The mission of STELLIM is to push the limits of the description of stellar evolution and stellar activity with milli-arcsecond resolution images. Moreover, as an interferometric imaging platform recombining 10+ apertures, it also paves the way for future astronomical facilities. In this contribution, we will present the design and the steps we took to progress towards a STELLIM demonstrator.