Lets say, we want to calculate the square of 25. The entire steps are illustrated in the following figure.
Steps for better understand:
Step 1 : Break the number such that 5 is on the right and the rest of the digit ( i.e., 2 in this case) is on the left.
Step 2 : Obtain the square of 5 which is 25. This forms the 2nd part of the answer.
Step 3 : Take the rest of the digit on the left and multiply by the next higher digit that follows it. Here, we need to multiply 2 by 3 which gives us 6 which forms the first part of the answer.
So, 252 = 625
One more Example: