In addition of fractions having similar denominators, just add the numerators and remain the denominator as it is. If the sum can be reduced to lowest terms then do so.
But in the case of fractions having unlike denominators, the least common denominator (LCD) should be get first then change the fractions into equivalent fractions with the LCD as the denominator. If the sum can still be reduced to lowest terms then do so.
If a proper or improper fraction is added with a mixed number, it is easier to convert the mixed number first into an improper fraction then use the steps and conditions mentioned above. Same situation applies with addends which are all mixed numbers.