Question

show me how to convert a decimal into a fraction in simplest form

Answer 1

drop the decimal place and use that resulting number as your numerator.
The denominator is a power of 10, were the exponent is the number of decimal places you had in your original number.
i.e. a 1 followed by as many zeros as you had decimal places

e.g. 0.123 = 123/1000

reduce the fraction to its lowest terms if possible

Answer 2

Alternatively:
N = .123
1000N = 123
N = 123/1000