How do I enter longitude and latitude into a tomtom one?

I have recently been asked to help my auntie with her GPS device as she has no clue how to operate it. After discovering how to use it I wrote an instruction manual. To check to see if it was all correct I had a go at geocaching. I thought it was excellent! The only GPS device is an in-car one, the tomtom one (4N00.003) How would I enter longitude and latitude numbers into this??