Reconfigurable Intelligent Surfaces (RIS) have been a hot research topic in recent years, being widely advocated to represent a promising technology for beyond 5G cellular networks. In this paper, we propose a modulation scheme called RIS-IM in which Index Modulation (IM) is applied using RIS. However, to overcome the strong channel correlation caused by the small separation distance between the RIS elements a precoding stage based on the concept of signature constellations is added at the transmitter. The precoding stage is shown to counteract the degradations caused by these correlations providing interesting gains. We further improve the performance by passive beamforming, optimizing the phase shifts of the RIS elements using Semidefinite Programming (SDP) to improve the receiver SNR. We then provide performance analysis for this IM scheme using computer simulations. A comparison of RIS-IM with Spatial Modulation (SM), which is another member of the IM family, is also included, showing that RIS-IM provides substantial SNR gains.