Basically, you just rub boiled linseed oil into the stock by hand. When manufactured, they just dipped the stocks into a vat of linseed oil. Just smelling the stuff reminds me of my high school ROTC armory, where we had about 200 M1s stored. Each cadet was responsible for cleaning his M1 weekly, and the protocol was to apply linseed oil to the stock as well. And those rifles were inspected at least once weekly by cadet officers during formal in-uniform inspections. After a while, the stock takes on a dull sheen that tells you you've done it right.