The singer and actress, 30, who got engaged to American football player Eric Johnson in November, said she will be dancing around her bedroom to "hopefully lose some weight" for the big day.
She said she wouldn't be dieting in the run-up to the wedding, adding: "No exercise is fun! I am going to dance around my bedroom and hopefully lose some weight - and lift some weights and do what I can!"
Jessica, 30, who divorced Nick Lachey in 2006 and was linked with singer John Mayer, has previously said she and her fiance were in no rush to marry.
She said earlier this month: "We've both been married before. We're enjoying our commitment to each other. We want to take our time."