How to merch in runescape?

I have 131mill cash. I never merched before, but I did some research on flipping- buy low, sell high. I'm a lvl 120 and pvm'ed my way to 131mill networth. I sold it all into gp. What do I buy, what do I do with it and how? really, how can i turn 131mill cash into 1 bill by merching.