We present TRing, a finger-worn input device which provides instant and customizable interactions. TRing offers a novel method for making plain objects interactive using an embedded magnet and a finger-worn device. With a particle filter integrated magnetic sensing technique, we compute the fingertip's position relative to the embedded magnet. We also offer a magnet placement algorithm that guides the magnet installation location based upon the user's interface customization. By simply inserting or attaching a small magnet, we bring interactivity to both fabricated and existing objects. In our evaluations, TRing shows an average tracking error of 8.6 mm in 3D space and a 2D targeting error of 4.96 mm, which are sufficient for implementing average-sized conventional controls such as buttons and sliders. A user study validates the input performance with TRing on a targeting task (92% accuracy within 45 mm distance) and a cursor control task (91% accuracy for a 10 mm target). Furthermore, we show examples that highlight the interaction capability of our approach.