Let's say we want to add 1/2 and 1/3. Since they both have different denominators, first we find the LCD:

Now that we have the LCD, we express the fractions with a common denominator:

Now that we have both fractions with the same denominator, we can add directly the numerators and keep the denominator:

We have that 1/2+1/3=5/6. Since 5/6 can't be reduced anymore, we have found the result.
To summarize, the algorithm to solve addition of fraction would be like this: