VBA (Visual Basic for Applications) in Excel allows you to create custom functions that extend the capabilities of the built-in functions. Here's a breakdown of the process:

1. Accessing VBA Editor:

- Press Alt + F11 on your keyboard to open the VBA Editor. This is a separate environment for writing code.

2. Creating a Module:

- In the Project Explorer pane (left side), right-click on your project (usually "VBAProject") and select Insert > Module. This creates a new module where you'll write your function code.

3. Define the Function:

- Start by typing the following line, replacing `MyFunctionName` with your desired function name:

```vba
Function MyFunctionName(parameters) ' As return_type
```

- `parameters` is a comma-separated list of arguments your function will accept (optional). Each argument can have a data type specified within parenthesis (e.g., Variant, Integer).
- `As return_type` (optional) defines the data type of the value your function will return (e.g., Long, String). If not specified, it defaults to Variant.

4. Write your code:

- Here, you'll write the VBA code that performs the desired calculation or manipulation based on the provided arguments. You can use various VBA statements, control flow structures (if/else, loops), and functions to achieve your goal.

5. Returning a Value:

- Once your calculations are complete, use the `Assign` statement to assign the final result to a variable with the same name as your function:

```vba
MyFunctionName = result_variable
```

6. End Function:

- Finally, close the function with the `End Function` statement.

Example:

Let's create a simple function to calculate the working days between two dates (excluding weekends):

```vba
Function WorkDays(startDate As Date, endDate As Date) As Long
' Loop through each day and count weekdays
Dim daysCount As Long
daysCount = DateDiff("d", startDate, endDate) + 1 ' Include both start & end dates
For i = startDate To endDate
If Weekday(i) <= vbFriday Then ' Check if weekday is Fri or earlier
daysCount = daysCount - 1
End If
Next i

' Adjust for weekends at the beginning/end
If Weekday(startDate) = vbSaturday Then daysCount = daysCount - 1
If Weekday(endDate) = vbSunday Then daysCount = daysCount - 1

' Return the number of workdays
WorkDays = daysCount - (daysCount Mod 7) \ 7 ' Adjust for incomplete weeks
End Function
```

7. Saving and Using the Function:

- Save your workbook (Excel file with the VBA code) as a macro-enabled workbook (.xlsm format).
- In your Excel sheet, you can now use your custom function like any other built-in function. For example, `=WorkDays(A1, B1)` would calculate the workdays between the dates in cells A1 and B1.

Remember: VBA opens up a world of customization in Excel, but it requires some programming knowledge. Start with simple functions and gradually progress to more complex ones as you learn VBA. There are many resources online and VBA documentation available to help you on your journey.