Can you tell me the sum of 123 by 0.2321? And also give me a very detailed description of the calculation.
I was a little confused by your question. 

If you mean what is the sum of 123 and 0.2321 then I would perform an addition. I would put the two numbers on top of each other, making sure that I line up the decimal points of each number and I would then add each column to give the result. 

The result of addition is 123.2321

If you mean what is the product of 123 by 0.2321 then I would perform a multiplication. I would break the multiplication down into three simpler multiplications and then add up the three results to give the final result. The calculation is shown below:

3 lots of 0.2321 = 0.6963
20 lots of 0.2321 = 4.642
100 lots of 0.2321 = 23.21

When the three answers are added together the result for the product is 28.5483

If you mean what is the result of dividing 123 by 0.2321 then I would perform a division. There are two methods available. You could either use long division or you could divide 1 by 0.2321 and then multiply the result (4.30848772) by 123 which gives the result for division of 529.94399
I'm sorry, I did mean to type the product of 123 by 0.2321. Can you write the answer to my question, with the proper significant figures as well?
The product of 123 by 0.2321 is 28.5483 to 6 significant figures.

There are multiple strategies that can be utilised to manually calculate the product of two numbers.

1. Use a calculator
2. Break down the problem into smaller multiplications as I have previously demonstrated
3. Use pen and paper multiplication techniques (https://en.wikipedia.org/wiki/Multiplication_algorithm)

With regards to the significant figures. I wasn't quite sure what you meant by 'with the proper significant figures'. I have stated above a number, with its corresponding number of significant figures.

The number of significant figures of a given value corresponds to the number of digits that are used to express it to the required degree of accuracy, starting from the first non-zero digit.
That is exactly what I was looking for, thank you very much! I will ask you if I find something unclear later on.