Some schools are educational institutions. Some schools are colleges. Some colleges are educational institutions. To represent the first premise of this argument, we need to show that something is in the area of schools that are educational institutions. So we need to place a lowercase x in both areas and draw a line between them.
Some schools are educational institutions.
Some schools are colleges. Some colleges are educational institutions. Clearly we have to represent the second premise in a similar way, except that we need to x-x the areas where schools and colleges overlap. Some schools are educational institutions. Some schools are colleges.