There are two kinds of boyfriend shirts:
1. The kind that is nicely styled and good for a casual day.
2. The kind that you wear while you're doing the Walk of Shame.
The other day, I'm pretty sure I saw someone with the latter...or the former, but with a pair of short shorts on that made it look like the latter. Another day, I saw someone with her blouse around her waist buttoned half way down to resemble a slit...or at least that's what I thought she was going for. So I have taken it upon myself to fuse their two looks together for a sort of "styling the boyfriend shirt for dummies" kind of post. In my opinion, it's not something I'd wear on a daily basis, but it's mildly less sloppy than the whole baggy shirt over the short shorts thing.
Though I'd say the most popular way to style this is by buttoning up the shirt. However, I find that this only works if your waist is small enough to button the shirt evenly. You should probably try to tie the sleeves in a neat bow, unlike me. I prefer to wear the unbuttoned shkirt to the side, but centering it works well too, especially if you're choosing to button the shirt. In these photos, I chose to leave the collar as is (sticking out a bit), but folding down the collar nearly into the insides of the shkirt makes for a more symmetrical look.
To experiment with this style, you're going to need a button-down shirt from a boyfriend, a husband, or a close-male-friend-who-you're-sorta-seeing-but-not-really-together-with. Or if you have none of the above, you can do as my singleton self did and buy a blouse 4 sizes too big.
*Edit: As per a few comments, yes, this is just a fancy way of tying a shirt around your waist when left unbuttoned.