For a very lightly flavored agua fresca, just mix the water and fruit together. Done! Obviously, if you want more fruit flavor, then use less water; if you want less flavor, then use more water.
I usually run my agua fresca through the blender, however. If you want the drink to be clear, strain the pulpy leftovers of the fruit after blending. If you’re using blueberries or oranges or other fruit with a skin, you’ll almost certainly want to strain it.
For some fruits, you can also choose to leave the pulp: it’s particularly great when making a melon agua fresca since it mostly disappears.
Serve over ice. Try some of the variations I’ve suggested or whatever fruits you like!