Find the LCD by listing the multiples of each denominator
Identify the smallest common multiple shared by all denominators
If denominators are small, use prime-factorization to build the LCD
Prime-factor each denominator into primes
Take the highest power of each prime that appears in any denominator
Multiply those highest powers together to get the LCD
If denominators share factors, include them only once at the highest needed power
Rewrite each fraction with the LCD as the common denominator by multiplying numerator and denominator by the missing factor
Use the LCD to combine fractions (add or subtract numerators, keep the LCD as the denominator)
Simplify the result by dividing numerator and denominator by their GCD if possible