If the denominators of the fraction (i.e. the bottom numbers) are the same, you can just add the numerators (i.e. the top numbers) together.

For example,

or

If the denominators of the fraction are different, you must first find the common denominator, then convert both fractions, then add the numberators when the denominators are the same.

For example,

The common denominator of 3 and 5 is 15.