To multiply $3.14$ by $2.5$ using a calculator, follow these simple steps:

1. Turn on the calculator: Ensure that your calculator is powered on and set to standard mode for basic arithmetic operations.

2. Enter the first number: Begin by inputting the first number, which is $3.14$. You can do this by pressing the buttons for ‘3’, ’.’, ‘1’, and ‘4’ in that order.

3. Select the multiplication operation: To indicate that you want to multiply, press the multiplication button, often represented by the symbol ’×’ or an asterisk ’*‘.

4. Input the second number: Now, enter the second number, which is $2.5$. You can achieve this by pressing ‘2’, ’.’, and ‘5’ sequentially.

5. Calculate the result: Finally, to obtain the product, press the equals button, usually marked with ’=‘. The calculator will display the result of $3.14$ multiplied by $2.5$, which is $7.85$.

This straightforward method can be applied to multiply any two numbers, offering a quick and efficient way to find the product without manual calculations.