To convert a ratio to a fraction, we just make the first number in the ratio the numerator and the second number in the ratio the denominator.
12 : 36 = 12/36
When simplifying fractions, you find factors that the numerator and denominators share, and then divide both by this factor.
The numbers 12 and 36 both share a factor of 12. Thus, to simplify 12/36, we should divide it by 12/12.
12/36 ÷ 12/12 = 1/3
Therefore, the ratio 12 : 36 as a fraction in simplest form is 1/3.
Hope this helps!