Most of the time, it is more difficult to draw on a three-dimensional object than on paper. To make things easier, simply use carbon paper to transfer a sketch of your design from paper to a three-dimensional object, such as a cup.
If you don't have carbon paper, you can also fully cover the reverse side of the piece of sketching paper featuring your drawing using a soft pencil. At the end, you can easily remove the pencil residues using an art eraser.