The way you make millions of FFXIV Gil is as follows

Ok, straight up, you're not going to find some sort of guide that tells you how to make millions a day with no effort. Even were that possible (I consider the gearing up and knowledge to tackle crafting as effort, even if the synths themselves wind up simple), nobody would tell you. It would be pointless.

I could go on a whole spiel about how money is essentially an abstract concept for goods and services and how the actual amount doesn't matter, but the short version is that how much people are willing to pay for something depends on how much they want it, and how annoying/difficult it is to obtain. Because of this, you can't just say "Oh, go make item X, it sells for Y ffxiv gil." because the moment you tell the world that, guess what? Everyone starts making that item. This means there's a lot of them out there, quite possibly more than anyone would possibly wish to buy at that price. So people lower the price, because people may not want to pay 5k for it, but they may buy it for 2k. But then supply is STILL exceeding demand, so the price drops more to try and grab even cheaper buyers, until eventually the item sells for so little that most other methods make more money for similar effort. The only times this doesn't tend to happen is when there's severe limits on production. Things like rare drops or time-gated events.

In the end, the way you make millions of Final Fantasy XIV Gil is as follows: Step one, have diverse methods. Have crafters, gatherers, and battle classes all well geared that you know how to play. Step two, pay attention to what people want, whether it's common raiding supplies or recent quest requirements, people need a reason to buy your crap. Step three is just to not spend money: Think about what you spend your ff14 gil on, and consider if it'd be less effort to make it yourself.