This can be done easily at any level really, even starting at Mudwater, however I'll start by giving an example from Neherrin River(lvl 10). Essentially all this method takes is a large fish-keeping net, management of time and return over investment, use of the in-game forward time function, and fishing for multiple days instead of just 1 day.

I feel that people in the know should help others, as to keep the game fun and to grow the community.

To me that made the game start to become monotonous and not very fun. I myself was having a tough time grinding long days for cash to buy new gear, repair gear, buy bait, and cover travel costs. Everything you need to know is included in the tool tips in the game, and the features used to do it were included in the game itsself by the devs to be used for the exact purpose of the 'method'. I don't feel that disclosing this method is neither 'game breaking' nor even really a secret. I saw a post on here recently talking about a 'secret' method of how to farm money.

Fishing planet money and exp farm