Steps to follow:
Open the Visual Basic Editor. In Excel, hit Alt+F11 if on Windows, Fn+Option+F11 if on a Mac.
Insert a new module. From the menu: Insert -> Module (Don't skip this!).
Create a Public
function. Example:
Public Function findArea(ByVal width as Double, _
ByVal height as Double) As Double
' Return the area
findArea = width * height
End Function
Then use it in any cell like you would any other function: =findArea(B12,C12)
.