To win, Downton fans must create a swish dinner party in their own homes, take photos and videos, and send them in for judging.
The Carnarvons are providing helpful tips on their website.
Lady Carnarvon said: “Decide what you are going to wear and how you are going to set the table. The latter can be a matter of contention.
“We have created some videos introducing various aspects of how we hold a dinner party here at Highclere.
“For example how my husband and Luis our butler choose the wines, or how Paul the chef and I discuss the menus, how Luis likes to set the table and how I create the flower arrangements.”
Four couples will be chosen to travel to the real Downton with friends, all expenses paid, and stay overnight at the castle with a grand dinner laid on.
Lady Carnarvon said she expects huge interest from the US, where the show is a big ratings success, but Britons can enter too.
She said: “We will pick the winner, and you and your lucky guests – a total of eight people – will fly over, dine and stay in the Castle.
“I read in the US press that Downton Abbey is the one place in the world where people most dream of staying.”
To enter the competition, which closes on February 28, go to ladycarnarvon.com/community/come-and-dine