If there are two or more denominators, each one can be split into its factors (if it has any). If each denominator is a prime (no factors) then the LCD is the product of all of them. The same applies if they have no common factors. For example: 2, 3 and 5 are prime so their LCD is 2*3*5=30; 2, 5 and 9 have no common factors so their LCD is 2*5*9=90.
The LCD of denominators with a common factor C means we can write each denominator as a*C (or simply aC), bC and cC where a, b and c are whole numbers. For example, if C is 2 and we have to find the LCD of 4, 6 and 10 we can write 4=2*2, 6=2*3 and 10=2*5. The LCD is 2*2*3*5=60. If we look at the LCD in terms of its factors we can see that there are two 2s, a 2 and a 3, and a 2 and a 5, so that means 2*2 (4), 2*3 (6) and 2*5 (10) will go into the LCD.
The LCD of a number and a multiple of the number is always the multiple. So the LCD of 2 and 4 is 4 because 2 goes into 4.
I hope this helps.