Answer by taking a quote from the following article:

The Blackfoot Confederacy, Niitsitapi or Siksikaitsitapi (nitmowmeko, meaning "the people" or "Blackfoot-speaking real people") is a historic collective name for the four bands that make up the Blackfoot or Blackfeet people: three First Nation band governments in the provinces of Saskatchewan, Alberta, and British Columbia, and one federally recognized Native American tribe in Montana, United States. The Siksika ("Blackfoot"), the Kainai or Kainah ("Blood"), and the Northern Piegan or Peigan or Piikani ("Apa'tosee" or "Poor Robes") reside in Canada; the Southern Piegan/Piegan Blackfeet ("Amskapi Piikani" or Pikuni) are located in the United States, where they are also known as the Blackfeet Nation. In modern use, the term is sometimes used only for the three First Nations in Canada. Historically, the member peoples of the Confederacy were nomadic bison hunters and trout fishermen, who ranged across large areas of the northern Great Plains of western North America, specifically the semi-arid shortgrass prairie ecological region.

The Niitsitapi main source of food on the plains was the American bison (buffalo), the largest mammal in North America, standing about 6 1/2 feet (2.0 m) tall and weighing up to 2,000 pounds (910 kg). Before the introduction of horses, the Niitsitapi needed other ways to get in range. The buffalo jump was one of the most common ways. The hunters would round up the buffalo into V-shaped pens, and drive them over a cliff (they hunted pronghorn antelopes in the same way). Afterwords the hunters would go to the bottom and take as much meat as they could carry back to camp. They also used camouflage for hunting. The hunters would take buffalo skins from previous hunting trips and drape them over their bodies to blend in and mask their scent. By subtle moves, the hunters could get close to the herd. When close enough, the hunters would attack with arrows or spears to kill wounded animals. The people used virtually all parts of the body and skin. The women prepared the meat for food: by boiling, roasting or drying for jerky. This processed it to last a long time without spoiling, and they depended on bison meat to get through the winters. The winters were long, harsh, and cold due to the lack of trees in the Plains, so people stockpiled meat in summer. As a ritual, hunters often ate the bison heart minutes after the kill. The women tanned and prepared the skins to cover the tepees. These were made of log poles, with the skins draped over it. The tepee remained warm in the winter and cool in the summer, and was a great shield against the wind. The women also made clothing from the skins, such as robes and moccasins, and made soap from the fat. Both men and women made utensils, sewing needles and tools from the bones, using tendon for fastening and binding. The stomach and bladder were cleaned and prepared for use for storing liquids. Dried bison dung was fuel for the fires. The Niitsitapi considered the animal sacred and integral to their lives.

how did they use bison?
The Niitsitapi main source of food on the plains was the American bison