Example: VENICE
Here we can see the bulk of my trading. I have merchants in Alexandria (forwarding to Venice), Crimea (forwarding to Constantinople), Constantinople (forwarding to Ragusa), Ragusa (forwarding to Venice), and Venice (collecting dat $$$). Also have one offscreen in Genoa, collecting there (with the 80% penalty
) since I don't really have better options for him.
I'm doing pretty serious work in Constantinople, helping to transfer about 2/3 of the value of the trade node out of there. I dominate Ragusa and send tons of $$ to my home of Venice, and control about 1/3 of the trade in Alexandria which I'm sending directly to Venice.
How am I doing this? Let's take a look!
In Alexandria, I have a small amount of trade power from provinces (Cyprus, Crete) and a slightly larger amount of trade power from a small group of light ships. This gives me the most power in the node, but as it's fairly crowded I have about 1/3 of the total trade power. The node isn't super rich, so I'm choosing to devote most of my light ships to other nodes. However, a long-term goal of mine over the next 100 years or so will be to make Alexandria a bigger priority, since it takes in trade from the Gulf of Aden which has connections to India and East Asia, and if I can use my fleet to start taking control of that whole trade route and direct it to Venice I'll be drowning in ducatzzz.
Constantinople has the Ottomans with tons of power, from both deploying light ships and having most of the provinces that trade in that node. I have only one province (Naxos), but deploy about 40 light ships to get myself on equal footing with them.
I have 20-25 ships in Ragusa, which gives me total domination over that node and allows me to forward the vast majority of trade there to Venice.
And of course, in Venice, you can see in the first screenshot I have 221 power in the node, which is about 80% of the total power there and lets me collect almost all the monies. I have no ships in Venice; it's all from the provinces in that node that I own, and from having lots of buildings (marketplaces, trade depots, canals, road networks) in those provinces which all boost the amount of trade power that those provinces will give me in the node they belong to.