Selkirk had become interested in the concept after reading Alexander Mackenzie's 1801 book on his adventures in exploring what is today the west of Canada. At the time, social upheaval in Scotland due to the introduction of sheep farming had left a number of Scots destitute, and Selkirk was interested in giving them a "better life" in a new colony he called Assiniboia.
He then purchased a controlling interest in the Hudson's Bay Company and set up the land grant. His idea (apparently) was to gain firm control of the area in order to take control of the West from the company's bitter rivals, the Montreal-based North West Company. With a colony in place the Métis trappers supplying the North West's fur traders, the Nor'Westers, would be displaced, cutting them off from areas further west.
The land included the watersheds of the Red and Assiniboine rivers, up to Lake Winnipeg, covering areas in what is today southern Manitoba and northern North Dakota. He sent out a small group of Scots in 1811 to the area, but they were forced to pause for the winter in York Factory. When they finally arrived in 1812 they built a fort, Fort Douglas, but by the time it was done the growing season was over and they hastily set about hunting buffalo for food.
When farming started the next spring, the results were less than expected and Selkirk had to ban anyone from taking food out of the colony. Of course this cut off the Nor'Westers, who relied on pemmican supplied to them by local Métis. They were so upset that they destroyed Fort Douglas and burned down all the buildings around it. The fort was later rebuilt and things settled down for a time.
Selkirk heard of the problems and sent out a new governor to take over. When he read a proclamation ordering the fighting to stop, the Battle of Seven Oaks broke out, Fort Douglas was destroyed for a second time, and the settlers were forced off their land. Selkirk then sent in a force of 100 Swiss mercenaries to enforce the peace, while also capturing the North West outpost at Fort William. This attempt worked, and peace was maintained until the Hudson's Bay Company purchased the North West Company and things settled for good. However it also left Selkirk almost bankrupt and was one of the reasons the two companies were forced to merge in 1821, thus ending the problems for good.
The colony was never particularly successful agriculturally, but the lure of free land added new settlers every year. However the Hudson's Bay Company lost interest in paying for settlement by the 1850s, and by the 1860s the Métis outnumbered the Scots. This led to a second round of fighting in 1870, and eventually, the creation of Manitoba.