High-entropy alloys (HEAs), introduced in 2004, represent a new class of materials composed of at least four principal elements in near-equiatomic proportions. Unlike traditional alloys, whose development has been guided by well-established thermodynamic and experimental databases, HEAs pose unique challenges due to their compositional complexity and vast design space. This chapter outlines practical strategies for designing HEAs, combining foundational principles with modern computational and experimental tools. It covers empirical design rules derived from thermodynamics and Hume-Rothery criteria, as well as advanced methodologies such as CALPHAD, density functional theory (DFT), molecular dynamics (MD), machine learning, and combinatorial synthesis. Each approach is introduced with a focus on its role in HEAs development and supported by practical examples. By providing a comprehensive overview of these tools and techniques, this chapter aims to serve as a practical guide for researchers and engineers seeking to design and optimize high-entropy alloys.