Check marketboard. Check the ore/metal button. See where there is demand. Go and mine that. Sell in various quantities. Crafters leveling will probably want 99. People just making gear or doing leves may only want 5 or 15. Price the smaller quantities a little higher than the 99-stack ones.

There is no set of instructions for making gil. You have to do research every day to see what is selling.

Also sometimes check the crystals button. Sometimes there is a shortage on crystals and you can turn a profit before the RMT step in.