- In each node, trade value is produced.
- Nodes can drain from other nodes and to other nodes. You need to send a merchant to drain to other nodes.
- Collection in your capital province is done automatically, otherwise you need to send a merchant, but in general it's not worth it since you'll collect at a penalty in a non-capital province.
Thus, if possible, the best idea is to use your merchants to steer trade from your colonies to your capital province, though if you have a weak place in the trade chain, you may have to collect elsewhere. Experiment with placement of merchants to see what works.
You can increase trade power in nodes with buildings, expanding to the provinces there, technology/ideas or sending light ships there to protect trade.