Achieving precise control over active metal dispersion in supported catalysts requires a deep mechanistic understanding of the synthesis process. In this study, we systematically dissect the individual steps of the incipient wetness impregnation (IWI) method to understand their influence on the physicochemical properties and catalytic performance of Ru/Al2O3 catalysts for CO2 methanation. By independently tuning the Ru precursor solution, drying conditions, calcination temperature, and subsequent post-calcination treatment, we isolate the effects of each parameter and reveal their mechanistic roles in controlling Ru nucleation, particle growth, and metal–support interactions. High dispersion of Ru nanoparticles (0.5–1.5 nm) was achieved at loadings of 0.5–2.5 wt%, resulting in catalysts with outstanding activity and stability. Among the examined steps, precursor concentration and solvent evaporation rate were identified as key factors governing Ru particle size and dispersion. Importantly, small Ru particles remained resistant to sintering even after calcination at 800 °C and long-term operation under harsh Sabatier reaction conditions, owing to strong metal–support interactions promoted by low-acidity precursor environments. Additionally, we demonstrate that residual chlorine species derived from RuCl3·xH2O precursors strongly inhibit catalytic activity if not removed. Ammonia-assisted post-treatment effectively eliminated surface chlorine, unlocking the full activity of Ru sites. This work provides the first comprehensive mechanistic analysis of individual synthesis steps in IWI, offering a pathway for rational catalyst design and scalable production of robust, high-performance Ru-based methanation catalysts.