Choose the Right Collar
When you're planning on letting the top few buttons undone, it's best to know what collar will look best unkempt. Camp collars are cool, of course. But you want to steer clear of larger spread or rounded collars. I suggest you find shirts you like with a button-down collar with visible or no-see buttons. These buttons (unless your shirt is cut from a light denim, then do without them) will help your collar keep its shape without needing to be buttoned together at all times; a somewhat-structured collar is still a requirement when the shirt is undone to keep you from looking like you're merely jacket-and-tie-less at the moment.
Tackle Your Cuffs
There are plenty of guides on rolling your sleeves—we have a particularly good one ourselves—but the best practice is to do what's comfortable. Like a good fade, keep it tidy and close, but don't worry about abiding by any military-like specifications. Your cuff roll should be unique to your liking and your needs.
Mind the Fabric
The best materials to wear casually, especially in the summer, are inherently light, breathable and somewhat durable. These prerequisites forge a short list of acceptable fabrics: cotton, linen, chambray and light denim; while rayon and silk both outmatch the aforementioned in breezy weightlessness, they don't hold shape nor do they last nearly as long. Your best bet is to seek out a comfortable, cottony shirt to break in and wear it to death.
Consider the Cut
You could wear a casual shirt tucked in, but most of the time you'll sport it untucked. Of course, some shirts are simply designed to be worn tucked in. If you wear said shirt untucked, it's both noticeable and sloppy—there's an obvious difference between longline and ill-fitting. Be sure the shirt you're buying, especially if it's online, is modeled untucked. If it isn't, check the length. Compare that length to shirts you own that you know look best tucked in. If they match, don't get that shirt with the intention of wearing it more casually.
Don’t Forget the Confidence
Perhaps most importantly, if you're unsure about your patch of chest hair that shows when you unbutton the top two buttons, don't try this. If you don't feel great about linen or if wearing a long sleeve shirt so casually feels sacrilegious, don't try this. All of the stylish men whom made this look so endearing had one thing in common: the boldness to loosen things up a bit. McQueen surely wasn't worried how many times his cuff was turned over on itself-despite being one of the best dressed men to ever live. Jagger didn't mind skipping more buttons than most men would have dared to. Let this look grow on you and the quirks and qualities of your style will come with time.