The way trade works is like this. Each trade node has a total trade value (gold) which is the sum of all the production of all the provinces in it, plus whatever's incoming minus whatever's outgoing. Now, you can either collect or transfer in each trade node with a merchant. You automatically collect in your home node so you don't need a merchant there (but it does give a 10% bonus if you put one there). If you have a merchant collect outside your home node he'll suffer a penalty.
So with that base info, this is how it basically goes. You want to do like a whirlpool and try to steer as much money as you can into your home node and collect there, so that's where trade steering comes into play.
The amount of money you collect or transfer in a node depends on your trade power in that node. Trade power comes from the provinces you own in that node plus if it's a marine node, the trade ships you have assigned there (with the Wealth of Nations DLC you can create a trading post in each node that adds trade power too). So to do anything of importance in a node, you need good trade power in it (about 60% is ok).
Let's look at the trade map for my current Venice game for example:
My home node is Venice, which is an 'end node' (no outgoing routes). Because of my manipulations, there is 60 gold available in Venice ('trade value'), where I have 78% trade power (about 50% from provinces and 28% from some trade ships) so I collect about 45 gold (60 * 0.78 give or take) base, plus my trade bonuses because of technology and ideas, I end up collecting some 80 gold there each month. The idea is to steer gold there from other nodes so I can collect more. In the image you see I have money coming in from Genoa (10.13 gold) and Ragusa (17.56 gold) (the red checkmark means I have a merchant transferring in that node and to which node it's transferring to). If I assign more trade ships to Genoa, my trade power there would increase and thus, more money would be transferred to Venice.
If we keep analyzing the map, Ragusa is usually very poor, but I also own Constantinople (70% power, from provinces and some trade ships), so my merchant is sending 20g from there to Ragusa. I also have some power in Alexandria and a merchant sending money (11.65) to Venice. I only have 47% power in Alexandria so my next available trade ships I'll probably send there, until I get to about 60%. I also have a merchant in Aleppo sending 10.5 gold to Constantinople. So you see how money is being funneled into Venice (Aleppo -> Constantinople -> Ragusa -> Venice, Alexandria -> Venice, and Genoa -> Venice). For this to work you need to have some good trade power in the nodes involved.
So in summary, look what rich nodes you have connected to your home node, send a merchant there to transfer to your node. Use trade ships (and/or warmongering) so you have some 60% power there. Later on when your fleet is very big, look into what rich nodes you can control so they transfer money to those, etc.
