When adding or subtracting fractions, you first convert the denominators to a common multiple (a number the denominators will all devide a whole # times into). You then convert the numerators by multiplying them by the same factor you multiplied the old denominators to make the new common denominator. Each numerator will be multiplied by a different factor. Then, with the converted fractions you add or subtract the numerator only and place the result over the common denominator.
Multiplying fractions is less complicated. Simply multiply the numerators to create the new numerator and the denominators to create the new denominator.
Dividing fractions is an contradiction in terms. Remember Keep-Change-Flip. Keep the 1st fraction in origional form, change the division sign to multiplication, and flip or replace with the inverse of the second fraction. The "flip" means the numerator becomes the denominator and the denominator becomes the numerator.
Operations with fractions tend to be one of the US students most challenging concepts. I have had good success with the web site www.aaamath.com. The tutorials and practice are very good.