"I think anybody who's gone through a pregnancy, after they have the baby, it's like, 'I need to do something about this,' " she tells PEOPLE, in its latest cover story. "For me, I really want to do something that is a lifestyle, because in the past, I've been known to yo-yo diet."
So, Simpson, who Tweeted "so excited to be part of the Weight Watchers family" Wednesday, signed on to work with Weight Watchers, a program she'd planned to do before she learned she was pregnant. When she got the good news, she stopped counting her points.
"I was like, 'I'm going to enjoy my pregnancy,' and I did," she tells PEOPLE. "I ate very healthy, but I did indulge some of my cravings – and that's okay because I know I can work it off now."
While her doctor hasn't cleared her to go to the gym yet, Simpson, who has yet to step on a scale, says she's excited to get back into shape and that her friends are going to do the program too, for support.
"There's not a goal weight – I just want to feel normal and fixate on inches," she says. "I really want to set small goals, so I'm constantly reaching goals and not looking at something so far ahead."
For more about Simpson's post-baby diet – and for exclusive photos of the new mom at home with baby Maxwell – pick up the new issue of PEOPLE, on newsstands Friday