Reconfigurable intelligent surfaces (RIS) hold the potential to revolutionize the wireless communications industry via the dynamic control of the wireless channels to help achieve high data rates, high spectral and energy efficiencies, and low error rates, towards meeting the advanced specifications of B5G. In this context, this paper proposes a practical channel model for RIS-aided MIMO communications which considers the effects of RIS configurations, operating environments, path loss, scattering, etc. With this comprehensive channel model, the probability density function and the cumulative distribution function of the received signal-to-noise power ratio are derived by utilizing a double generalized K distribution, and closed-form expressions for the system's error rate, outage probability, and channel capacity, are obtained. The analysis highlights the outperforming of the RIS-aided communications in indoor environments as compared to outdoor use cases due to the presence of less scatterers in the former.