We give an overview of the measurement process starting from observations at the telescope and the construction of a map of the large-scale distribution of galaxies, to the estimation of cosmological parameters. Along the way, we will show examples from the Canada-France- Hawaii Telescope. With the aid of VIPERS spectroscopic data, we present measurements of the galaxy clustering pattern at redshifts 0.5-1. From the power spectrum we derive constraints on the dark matter density as well as neutrino mass. We will highlight the challenges posed by complex galaxy physics and how these might be overcome to arrive at precision cosmology.