Euclid is an ESA mission dedicated to understand the acceleration of the expansion of the Universe. The mission will measure hundred of millions of galaxies in spectrophotometry and photometry in the near infrared thanks to a spectro-photometer called NISP. This instrument will be assembled and tested in Marseille. To prepare the on-ground test plan and develop the test procedure, we have used simulated PSF images, based on a Zemax optical design of the instrument. We have developed the analysis tools that will be further used to build the procedure verification. We present here the method and analysis results to adjust the focus of the instrument. We will in particular show that because of the sampling of the PSF, a dithering strategy should be adapted and will constraint the development of the test plan.