To break a broody, get a wire cage (or some other enclosure can work, I use my brooder or a small dog exercise pen) and put the chicken in there with food and water. She stays in there around the clock for about 48 hours, or until you start seeing a decline in broody signs (flattening like a pancake, hissing, puffing, constant tik-tik-tik sound), then you can test her by letting her out and seeing what she does. If she heads back to the nest at any point, back in the cage she goes, for another 24 hrs, then test again.
If you cannot safely keep the bird caged around the clock (no room, no run or you don't feel run is predator proof) then take her out at night and put her directly on the roost, then cage again next morning.
If you break early on, it usually takes 2-3 days to break them. Delaying it may make it take longer, as the bird is more set on brooding.