How do I make someone enter information in a cell

I have created a worksheet for my employees to track their daily activies.
Some cells are optional other must be filled out. How do I force these cells
to be used?

I like to use an adjacent cell that is formatted in big, bold red letters:

=if(a1<>"","","<-- Please complete this cell!")

This would go in B1.

Vicky T wrote:
>
> I have created a worksheet for my employees to track their daily activies.
> Some cells are optional other must be filled out. How do I force these cells
> to be used?

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Dave Peterson