I'm looking to write a VBA function that will create an error pop-up to
appear when a user enters in data that does not match the expected
value. The pop-up only needs to give an OK option to the user as it's
just going to be used as a warning that the figures don't match.

I have very limited (aka almost none) VBA writing experience so the
less technical, the better.

Thanks in advance for any help.