I understand prime numbers and I understand factors. 
Writing a number as a product of its prime factors can be a useful way to analyse a number.
It can can be done 'freely', or in a more structured way.
Use your 'general knowledge' of tables to spot factors of the original number (except 1).
Repeat the process for each of these factors, until you can go no further.
This will happen when all of the factors are prime numbers.
 or... 

So, writing the factors in numerical order:  140 = 2 × 2 × 5 × 7 
or (even better):  140 = 2^{2} × 5 × 7 
Notice that it doesn't matter which factors of the original number you spot first.
You always end up with the same prime factorisation.
Example: Find the prime factorisation of 1386
1386  ÷  2  =  693 
693  ÷  2  2 is not a factor of 693. Try 3...  
693  ÷  3  =  231 
231  ÷  3  =  77 
77  ÷  3  3 is not a factor of 77. Try 5...  
77  ÷  5  5 is not a factor of 77. Try 7...  
77  ÷  7  =  11. 
Since 11 is prime, we can stop. 
So: 1386 = 2 × 3^{2} × 7 × 11
This is a better method to choose for larger numbers, or when it is hard to spot factors.