Deep eutectic solvents (DESs) have recently gained attention due to their tailorable properties and versatile applications in several fields, including green chemistry, pharmaceuticals, and energy storage. Their tunable properties can be enhanced by mixing DESs with cosolvents such as ethanol, acetonitrile, and water. DESs are structurally complex, and molecular modeling techniques, including quantum mechanical calculations and molecular dynamics simulations, play a crucial role in understanding their intricate behavior when mixed with cosolvents. While the most studied cosolvent is water, in some applications, even a small content of water is considered a contaminant, for example, when the processes of interest require dry conditions. Only quite recently have modeling studies begun to focus on DES mixed with cosolvents other than water. This tutorial provides the first comprehensive overview of these studies. It highlights how modern molecular modeling increases our understanding of their structural organization, transport properties, phase behavior, and thermodynamic properties. Additionally, case studies and recent developments in the field are discussed along with the challenges and future directions in molecular modeling of DES in cosolvent mixtures. Overall, this review offers valuable insights into the molecular-level understanding of DES-cosolvent systems and their implications for designing novel solvent mixtures with tailored properties for various applications.