Buried waveguides with nearly symmetrical refractive index profile and high homogeneity were obtained by applying the reverse-proton-exchange technique to MgO doped stoichiometric lithium tantalate, a promising nonlinear material due to its low coercive field and high damage threshold. By characterizing several samples fabricated under different experimental conditions, we identified a fabrication procedure in which the annealing and the reverse-exchange processes are performed at the same temperature, and the diffusion of hydrogen ions towards the substrate is negligible during the burial step. These fabrication conditions are simpler than the conventional ones used for lithium niobate. Accurate empirical laws were found, relating the fabrication conditions to the optical parameters.