Instead here's a few tips on how NOT to make gil.
1. playing the marketboard: that's just gambling, many people will tell you it's a skill based system but in reality it's just luck and dedicating hours a day
2. minimum wage jobs: farming crystals or selling drops from old trials, very labor intensive and feels like you're working instead of enjoying a game
3. tricking people into buying your overpriced stuff on the marketboard: same idea as 1, it's a good way to burn yourself out
4. seasonal work: as the name suggests, it's seasonal only, sell food and potions and materia, very labor intensive and very easy to burn yourself out, and not sustainable.

The hard answer is that earning a fortune is a long term project, and to not burn yourself out you need to find passive ways of making gil as well as just not spending any.
For that you have either submarines, or simply not caring about gil and it being a byproduct of you trying to be self sufficient and farming everything yourself.