You start with a few acres - general poor land that can only grow livestock. You raise a few cows/hogs/... per year on the land but you go to work in the nearest town doing something else full time to live. In 5 years you have a reputation with the bank and some of the land paid down so you can buy another lot when it goes on sale thus expanding the farm - but you are still working full time somewhere else to get money. After 20 years of this eventually you build up enough that you can support yourself farming - much of the land is bought at yesterdays prices - but it will be many more years before you can make more than if you stayed working.

Or you marry someone who will inherit land - this is often the best way in.