best way to make soil more acidic?

Get a soil test first, cheap kits from any garden centre, so you know how far you need to go. Elemental sulphur is the standard way - it's slow, takes months, so add it well before planting. For blueberries and the like, digging in pine needles or composted bark helps too. Watch the coffee grounds tip, used grounds are close to neutral and don't do much.