How do you make linseed oil dry?

It all depends, I suppose, on your usage. The 100% safest way is just to buy it at the store... never try to make it yourself by boiling raw linseed oil as you'll no doubt wind up with a pretty good fire on your hands.

Today's "Boiled Linseed oil" is mixed with chemical agenyts that cause it to dry more quickly than raw linseed oil. Heating raw linseed oil (under laboratory conditions!) causes it to polymerize and oxidize, making it thicker and shortening its drying time. Again, kids, don't try this at home!