To find the LCM we break each number down into its factors:
2 has no other factors (ignore 1 when breaking down any number into factors); 4=2*2; 6=2*3.
Now write down the factors in a line as multipliers, omitting duplicates between different numbers:
2*2*3. If you look at this line you can see a single 2, 2*2 and 2*3. These are the numbers 2, 4 and 6. So the line-up gives us the LCM=2*2*3=12.
All LCMs can be found this way.