You grant certain towns the status of 'royal burgh'. Many of these are along the North Sea coast or on the Firth of Forth, well placed for trade with northern Europe. You give privileges to men called 'burgesses'. They and only they are allowed to trade overseas. They don't pay local tolls, and only pay you for the goods they export. Trade continues and thrives.
Now you've introduced royal trading burghs, a market economy develops, and goes from strength to strength. People become involved in manufacturing goods and overseas trade flourishes.
