We teach students to find equivalent fractions by using models, and then by multiplying or dividing by one whole.
Whenever you multiply or divide a number by 1, the value of the number stays the same. This is the same with fractions. Whenever you multiply or divide a fraction by a form of one, the value of the fraction stays the same.
For example, 1/2 x 2/2 = 2/4, so 1/2 and 2/4 are equivalent fractions.