Curves come in varieties for every need. For loose, energetic curves, freehand drawing provides the most vigorous results. Freehand drawing is more expressive. For complicate curves, but ones that have variety and sharp precision, a French curve tool is best. These curves are excellent for design projects. Curves in things like engineering drawings, or a design based on shapes, are well-executed using templates, which come in a wide selection. Pick the method for your task that provides the attributes you need.
Draw curves freehand with smooth lines. Place the paper upright. Tack it to a wall, or pin in to cardboard. You will get freer curves than if you work flat and your drawing arm elbow is on the table. Sweep your arm freely over the paper, prior to drawing. This will loosen you up. Begin with your pencil hovering above the paper. As you sweep your drawing hand down in a flowing curve, allow it to touch the paper. Top to bottom produces looser curves. Try side to side as well. Let your body move, too. Drawing is physical. This method produces smooth, gestural curves.
Create complex, clean curves with a French curve. Lay it on your paper. Trace around one of the edges. If that curve is sufficient, stop. For more complexity, keep the pencil point in place. Remove the French curve, then select a different part of the many curves on the french curve. Lay it beside the pencil point. Adjust the french curve. You want the line you already drew to smoothly meld with the new part of the French curve. Continue tracing around the curve. Repeat as necessary. You can create very complicated, pleasing curves this way, with great precision.
Use a template for curves of particular shapes. An ellipse or circle template, available at design or art shops, is used to make a crisp, perfect arc or elliptical curve. Pick the size you want. There will be a variety on the template. Lay the template on the paper. Trace around the inside, drawing an arc or section of your selected shape.