When adding or subtracting fractions with the same denominator, you simply add or subtract the numerators and keep the denominator. Examples:
If you have unlike denominators, then you need to find a common denominator. If the greater denominator is a multiple of the lesser, than you only need to adjust one fraction. You change it multiplying the denominator by whatever you need to make it the same. Whatever you multiply the denominator by, you have to also multiply the numerator by to keep it an equivalent fraction. The example is addition, but subtraction works exactly the same.
If the greater denominator is not a multiple of the lesser, than the easiest way to convert them to a common denominator is by multiplying one fraction by the other denominator and vice versa. Remember, we have to multiply both the numerator and the denominator to keep them equivalent. The example is subtraction, but addition works the same.
The last step I show there, the ones circled in green, is showing taking the answer and converting it into its simplest form.