Summarize:

Some coffee beans undergo processing with steam or solvents to remove some of their acidity. Other coffee beans are naturally low in acid because of the area of the world that they grow in. Start with a coffee that's designed to be low-acid if you're concerned about acidity.  Coffees that are processed to remove the acidity are often marketed as “mild” or “stomach-friendly.” Many coffees grown in Hawaii, Sumatra, Brazil, India, and the Caribbean are naturally lower in acid. Dark roast coffees, such as Italian and French roast, are usually lower in acid than light or medium roast varieties. That's because coffee undergoes chemical changes as it roasts, so the longer that it does, the more acid that is removed. Choose a dark roast coffee and make it using any method that you prefer.  Dark roast coffee also causes less acid to be secreted by the stomach than light or medium roasts. If you're not used to drinking dark roast, use a smaller amount than you would of your usual light or medium roast until you're accustomed to the taste. If coffee beans are ground too finely, too much of the acid may be extracted during the brewing process. To reduce the acid in your coffee, opt for a coarser grind, which isn't likely to be over-extracted when you're brewing your coffee. You may want to try a medium ground coffee to see how it affects your stomach. However, avoid fine and extra fine ground coffees.
Select coffee that's advertised as low-acid. Opt for a dark roast coffee. Use coarsely ground coffee.