Let me teach you how to wear them
Update your jeans and T-shirt look. The easiest way to wear a boyfriend blazer is over a simple T-shirt or button-up and fitted jeans. Because the jacket is cut larger than most, wear at least one fitted article of clothing, whether it be a shirt or pants, so you don't look shapeless. For the most feminine look, roll or push up the sleeves to expose your wrists.
Slip on a skirt. Wear your blazer over a full skirt or, if you're really daring, don it with a miniskirt so that it almost looks like a business suit. Wear tights or leggings for a more modest and streamlined look, especially if the blazer is long enough to cover the hem of the skirt.
Add a belt. If the oversized look is too overwhelming for your frame or you simply don't like it, try wrapping a belt around the blazer to fit it to your waist. A long, colorful scarf could also work for defining your waist. You can pair this look with either fitted jeans or wide-leg pants. A full skirt or pencil skirt would also look nice with a belted blazer.
Throw on leggings. If you have shapely legs, consider wearing the blazer over a simple T-shirt and dark leggings. Add ankle boots for a trendy look or wear flats or funky sneakers for a different look. Belt the blazer to give your waist more definition and look for a longer blazer that will still cover your bottom so you don't reveal too much.
Continue the menswear trend. Try wearing your oversized blazer over a plaid shirt and jeans for a boyish look. To avoid looking too masculine, however, wear cute heels and delicate jewelry. Also, step up your makeup and hair a bit so that you still look girly.
Use it in place of cover-up. A sleeveless sheath dress or jumper looks great with an oversized blazer over it. Be sure to add cute heels or wedges to dress up the look, since the blazer adds a casual vibe.
Go long with your shorts. The boyfriend blazer can easily dress up a pair of cuffed shorts or cropped pants. Just be sure to wear a belt or other accessory to define your shape, especially if you wear wide-leg shorts, and wear heels to look taller.
Wear a scarf or big necklaces. Button the blazer and wrap a colorful scarf around your neck to dress up the outfit. A long, chunky necklace or a few thick strands of beads can also make your look more sophisticated.