Custom add-ins are a great way to extend the functionality of Excel. There are a few different ways to create custom add-ins, but the most common is to use Visual Basic for Applications (VBA).
Creating a custom add-in is relatively simple. First, open the workbook in which you want to create the add-in. Next, press Alt+F11 to open the Visual Basic Editor (VBE). In the VBE, select Insert > Module. This will insert a new module into your workbook.
Now, you can start writing your code. There are a few things to keep in mind when writing code for an add-in. First, all code must be placed within a subroutine. Second, all code must be executed from the Worksheet_Activate event. This event fires whenever the worksheet is activated, which ensures that your code will run every time the worksheet is opened.
Once you have written your code, you need to save the workbook as an Excel Add-In (*.xlam). To do this, simply go to File > Save As and select *.xlam from the drop-down menu. That's it! Your custom add-in is now ready to use.
To use your custom add-in, simply open Excel and go to File > Options > Add-Ins. In the Manage drop-down at the bottom of the dialog box, select Excel Add-Ins and click Go. This will open the Add-Ins dialog box. From here, you can browse for your *.xlam file and select it. Once it's been added, your custom add-in will be available every time you open Excel.