1. You can pair it with pants and loafers, if you want it casual. Will look better if the color's appropriate with the flannel's color. You might want to tuck it in or out. 2. Wear a sweater and let the collars show, I swear it looks hot. 3. For smart casual, pair it with a blazer and denim pants and leather shoes.
Of course, the most important, wear it with confidence :)
Some jeans, black or dark blue, depending on the color of the shirt (I don;t like light colored jeans). Roll the sleeves up to you elbows, not above them, and roll them like if you were folding them, not sloppy. Now that it's cold, wear a jacket, unzipped.
I usually wear them with black jeans, vans, and a leather jacket; I look great in that.
I usually wear an oversized flannel with leggings and booties. I get a lot of compliments
Depending on what color the flannel is maybe some nice dark blue jeans, or just plain blue ones or black. You can throw a cool coat over it and maybe a beanie if that's something you'd like , and boom you've got a little style going on.
Flannels (on guys) look the best with jeans, in my opinion. Also, you can button it up all the way and roll them up on the arms or leave it unbuttoned with a simple, white t-shirt under
For a guy, get one that fits well, not oversized (but not tight like a dress shirt). An undershirt is common, go for something other than white if you've got it. One option is to wear it like a loose jacket, and not button it or tuck it in. Jeans or potentially corduroys for pants. If you tuck it, go for an unusual belt (and buckle) if you're looking to stand out a bit. I have a bronzed leaf buckle that my father bought in the early 70s which is the perfect accompaniment. Boots are basically a must. Normally the collar will need to be ironed to look decent.
Flannels never go out of style, so it's a shirt you'll be wearing for a long long time if you have a decent number of other shirts in your wardrobe and don't wear it out. :) Good luck.