>>2067475
Fundamentally, the key to trade is expanding your empire. So let's say you expand into a node; you now have some trade power from the new provinces, but it's not a lot. In order to maximize your power, you should find a state that has at least 4 provinces and a trade center and turn it into a trade company. You then buy a Company Depot (the building on the right) for that state. This usually gets you to the 50% threshold to gain a merchant, which you can use to either collect or transfer (there's no harm in experimenting with your merchant usage for a couple of months to see which makes you more money). Importantly, though, this also significantly boosts the goods produced for all provinces in that node that are NOT in a trade company.
And that is the basics of it. You can also purchase the building on the left to further boost your income. Lastly, since trade companies are the key to generating more trade, your can change your actual capital (not your trade capital) to a different subcontinent to get more trade companies.