“Serge [Gainsbourg] used to say, ‘Oh, lick your lips and toss your hair back,’ ” recalls Jane Birkin, the singer and actor, of the fashion advice her former partner used to give her. And one day, “I said, ‘No, I won’t do that anymore. I want not to notice what I look like at all!’ ” That was when, about 40 years ago, she exchanged her minidresses and hot pants for the low-key, boyish charms of baggy trousers and men’s shirts—ironically, a style that has become even more imitated.

That uniform is the genesis of a new collaboration with A.P.C., the beloved label started by Jean Touitou that’s been defining uncompromising casual French clothes for more than 30 years. This is not an instance of two friends scheming but rather two eccentrics meeting their match, as they explained in a joint phone call. “We are not friends!” Touitou yells, breaking into laughter, although “obviously we have a lot in common.” It was Touitou’s idea to work with Birkin. A mutual acquaintance connected them, bringing to fruition the latest of what A.P.C. calls its “interactions.” The last one with an actress, Birkin notes, was with Catherine Deneuve. “I thought, that’s really chic to be doing it after Deneuve!”

“That anyone would want to wear things I did or look like me, I was amazed!”

The exacting, tight collection looks like what Birkin, now 75, wears daily: jeans, trousers, T-shirts, sweaters, a coat, sneakers, a straw bag, and a waist bag. Taking cues from what she owns, “I just did a few drawings of what that uniform could be if it was summer, winter, and spring,” Birkin explains. Each item is an improved version of one of her favorite things, like workmen’s trousers she began wearing “because I was too fat to get into jeans,” and “which we’ve done beautifully here,” in a soft cotton with the pockets she prefers to stuff with keys and coins when she wants to avoid carrying a bag. “Everything that I’ve found, it’s been nearly all right, but I thought I could perhaps make it even more all right,” she says.

“She wouldn’t need us to make a perfect wardrobe,” Touitou adds. But there is “this very slight, slight, slight difference between something that is too much and something that’s not quite enough, and this is what we’ve been doing.” Touitou feels that the project took him back to A.P.C.’s radical roots as a creator of exceptional basics and says, “We’ve been working on doing these pieces in quite the right way.”

Birkin thinks obsessively about the smallest peculiarities of what she wears and how the subtlest change might make something fine into something indispensable. She has always wanted the neck of a T-shirt or sweater to scoop a bit lower because it then drapes just off the shoulder, “which is rather attractive.” Tennis shoes are lined with faux sheepskin because she gets a bit chilly in sneakers during the winter and “I always think it looks horrid when you take your socks off and you’ve got that mark around your ankle.”

Birkin, a constant on designer mood boards and the namesake and inspiration of Hermès’s most famous bag, has never designed a line before. Her daughter Lou Doillon kept pressing her, showing her the frenzy that her image continues to inspire on social media. “That anyone would want to wear things I did or look like me, I was amazed!” She thought her clothes “were rather boring. Nice, but boring.” Perhaps the reason Birkin’s look endures is that she is one of the first stars we came to know through her most casual, unstyled clothes—not in gowns or glamour shots but in T-shirts and jeans.

“It’s a whole art to be invisible,” Touitou muses. “To be noticed for what you actually do or say or write and not for your appearance.” When image represents much of what we’re supposed to be, he says, what he and Birkin wanted to do instead “was be invisible and yet very elegant.”

The collection goes on sale September 12. A version of this story appeared in the September issue of Harper’s Bazaar.

Rachel Tashjian is the Fashion News Director at Harper’s Bazaar, working across print and digital platforms. Previously, she was GQ’s first fashion critic, and worked as deputy editor of GARAGE and as a writer at Vanity Fair. She has written for publications including Bookforum and Artforum, and is the creator of the invitation-only newsletter Opulent Tips.