Managing a workbook using only the standard functions of the spreadsheet may be problematic. To manage your procedures and custom functions, you can use the Visual Basic Editor: Go to Tools > Macro > Visual Basic Editor or press Alt + F11 simultaneously.
The following window "Project - VBAProject" shall be displayed, if it is not present, go to View > Project Explorer or simply press Alt + R.
In this window, you will see your file and you can expand its tree as explained below:
the "Microsoft Excel Objects" folder including:
one a line for each worksheet - for specific procedures of each sheet.
the "ThisWorkbook" line, for procedures workbook.
the "Sheet" folder, if you have created forms, with the list of forms.
the "Modules" folder, for one line created for each module.
If you used the macro recorder to record your procedure, you will find it in a module (with the name you gave it).
If you want to introduce a procedure from an online source or forum, double click on the appropriate line and paste the it.
To create a module or a procedure, use the Insert menu.
A custom function must be created in a standard Module in order to be called from a sheet.