It all falls back to the definition of modulus: It is the remainder, for example, 7 mod 3 = 1. This because 7 = 3(2) + 1, in which 1 is the remainder.
To do this process on a simple calculator do the following: Take the dividend (7) and divide by the divisor (3), note the answer and discard all the decimals -> example 7/3 = 2.3333333, only worry about the 2. Now multiply this number by the divisor (3) and subtract the resulting number from the original dividend.
so 2*3 = 6, and 7 - 6 = 1, thus 1 is 7mod3