Here's how to add all numbers in a specific sheet with the SUM function in Excel:

Method 1: Using a cell reference

1. Choose a cell for the sum: Select an empty cell where you want the total to be displayed.
2. Enter the SUM function: Type `=SUM(` in the formula bar.
3. Reference the entire sheet (optional): If you want to sum all numbers in the entire sheet, you can simply enter the sheet name followed by an exclamation mark (!) within the parentheses.
For example, `=SUM(Sheet1!)` would sum all numbers on a sheet named "Sheet1".

Method 2: Using a range reference

1. Choose a cell for the sum: Similar to method 1, select a cell for the total.
2. Enter the SUM function: Type `=SUM(` in the formula bar.
3. Define the range: Click and drag to select the range of cells containing the numbers you want to add. Alternatively, you can type the cell range manually (e.g., A1:A10).
4. Close the parenthesis: Type a closing parenthesis `)` to complete the formula.

Examples:

* To sum all numbers in column A on Sheet1: `=SUM(Sheet1!A:A)`
* To sum all numbers in the range B2:D5 on the current sheet: `=SUM(B2:D5)`

Press Enter: Once you've defined the range or referenced the sheet, press Enter. The SUM function will automatically calculate and display the total of all the included numbers.

Tips:

* You can use this technique to sum numbers across multiple sheets as well. Simply separate the sheet names with commas within the parentheses. For example, `=SUM(Sheet1!A:A, Sheet2!B:B)` would sum cells A:A in Sheet1 and B:B in Sheet2.
* The SUM function can handle various data types, including numbers, currency, and percentages. However, it will ignore text characters or error values.