"Learn to embrace your imperfections," the star wrote. "I should probably say, don’t mess with your boobs. All those years I denied it -- stupid. A sign of insecurity. Just celebrate what you’ve got."
Though excerpts from the letter were originally published in the October 2016 issue of the magazine, the entire letter didn't appear online until last week.
On dealing with fame, she wrote, "You are going to be very, very famous, both for the band you form and because of the man you marry, and then later for a fashion business you will launch in your own name. You will get used to fame. Although you cannot set a price on losing privacy, you will learn to use celebrity to your advantage. For good things. For charity."
Beckham, who has been married 17 years to former soccer star David Beckham, also advised her younger self on how to stay happily married.
"Have patience. Bite your tongue. Be supportive. And preserve a bit of mystique," she said. "Never let yourself go completely (at least brush your hair, clean your teeth, have a bit of a brow going on because you will always want him to look at you and feel attracted)."
As for motherhood, the mother of four wrote, "Once you are a parent, you worry. And you are going to have four, so that’s a lot of worry! ...Your children will always come first, but never forget who you are and what you want to achieve."
She also warned about the "bags under your eyes" that come with children and that a good pair of sunglasses will become her friend.
"You will never be one of those people who just roll out of bed. Wear sunglasses a lot. Even inside. Especially at airports," she wrote. "They turn a nothing-outfit into something quite pulled together and cool. You are going to really like Aviators. (Then one day you will develop your own!)"