To master common size analysis, follow these steps:

1. Understand the concept: Common size analysis is a technique used to analyze financial statements by converting all the line items into a percentage of a common base. This helps in comparing the financial performance of different companies of different sizes.

2. Choose the financial statements to analyze: Common size analysis can be done on the income statement, balance sheet, and cash flow statement. Choose the statement that you want to analyze.

3. Determine the base: The base is the total of the statement. For example, if you are analyzing the income statement, the base would be the total revenue.

4. Calculate the percentages: Divide each line item by the base and multiply by 100 to get the percentage. For example, if the revenue is $100,000 and the cost of goods sold is $60,000, the percentage for cost of goods sold would be (60,000/100,000) x 100 = 60%.

5. Analyze the results: Once you have calculated the percentages, analyze the results to identify any trends or patterns. Look for any significant changes in the percentages over time or compared to other companies.

6. Use the results: Use the results of the common size analysis to make informed decisions about investments, financial planning, and business strategy.

7. Practice: The more you practice common size analysis, the better you will become at it. Try analyzing different financial statements and comparing different companies to improve your skills.