Find the lowest common denominator of the two fractions if the denominators are not the same. List the first several multiples of the first denominator, then list the first few multiples of the second denominator. Find the first number common to both sets; this is your lowest common denominator. For example, if the two fractions were 3/4 and 2/3, the lowest common denominator would be 12, the first multiple shared by both denominators.
Multiply the first fraction in order to reach the LCD. In the same example, the lowest common denominator is 12, so the top and bottom of the first fraction must be multiplied by 3 since its denominator is 4. Multiply 3/4 by 3/3, ending up with 9/12.
Multiply the second fraction in order to reach the LCD. If the lowest common denominator is 12, as in the example, multiply 2/3 by 4/4 to end up with 8/12.
Rephrase the problem with your new fractions. For example, 3/4 -- 2/3 has now become 9/12 -- 8/12.
Combine the subtraction problem over one denominator. In this case, 9 -- 8 would appear over 12. Subtract the top two numbers to determine your final numerator. Place this above the same denominator as before.
Simply the fraction if possible. In this example, 1/12 cannot be simplified and would be the final answer.