Use the sliders, and the arrows (arrow keys also work) to change the nature. To find a Pokemon's base stats, go to Bulbapedia and find the base stats section on the Pokemon's page. You can only have a maximum of 508 EVs (effort value points) on a Pokemon, though there's no limit on IVs (individual value points) aside from the max of 31 for each stat. Use fullscreen for precise slider movements.
Reference: https://bulbapedia.bulbagarden.net/wiki/Statistic#Permanent_stats I know there's a million projects like this, but I found the math on this page interesting so I made a project out of it.
