It's not very tough.

1) You have to build to the Keep level. Pick a province that has a tradeable good.

2) Build a port, then a dockyard.

3) Now you can build ships. Start queing them up.

4) As each ship goes out, slide the ship to an adjacent water space towards foreign ports.

5) In the meantime, in your provinece, build a trading post.

6) When your ships form a line between your port with a trading post (or higher) and a foreign province with a port that does not have that trade good as well, you get the income for selling them that good.

7) Keep building more ships to increase the number of foreign ports you reach.

8) Keep building more domestic ports and trade buildings (and link them with ships too) to increase the number of goods you have for trade.

Every province you have that has a good will sell that good, even if it's multiple times. Example: The Danish build a port in Denmark. You're England. You have ports & trading posts in Wessex, Mercia, and North Umbria, all linked by ship to Denmark. All 3 provinces will sell wool to Denmark, not just 1.