Best way from Gatwick to Stansted

Gatwick and Stansted are opposite sides of London. There is no direct route.
"Gatwick Express" train to London Victoria station, Underground (circle line) to Liverpool station, then train direct to Stansted.
If you go to the ticket office at Gatwick station ticket office and ask for a ticket to Stansted they will explain to you exactly what to do.

Victoria Line to Tottenham Hale then change to the the Stansted Express there is easier than via Liverpool Street. There is an infrequent coach service, but unlike Gatwick to Heathrow this is slower than the cross London route already described.

The budget way would be the train to Victoria then a coach.

Thank you for your quick responses.

If your flight is convenient for the direct coach between the two airports (see www.nationalexpress.com for times), you might still want to use it for the convenience of not having any changes. There will be at least one long escalator ride and some stairs whichever rail/tube route you pick.