<p>The stabilization of metal catalysts remains a major challenge in heterogeneous catalysis, particularly for single atoms and clusters that operate under demanding industrial conditions where sintering and agglomeration often lead to substantial deactivation. Here, to address the issue, this protocol presents a general method for synthesizing nanoisland catalysts in which nanoscale oxide islands are isolated on high-surface-area substrates to confine and stabilize metal species. The method uses strong electrostatic adsorption to deposit oxides such as CeO_<i>x</i>, LaO_<i>x</i> and InO_<i>x</i> onto substrates such as SiO₂ and Al₂O₃, resulting in small-sized, high-density and uniformly distributed nanoislands. Then, transition-metal precursors, including those of Pt, Pd and Ru, are introduced in a controlled manner such that single atoms or clusters are positioned preferentially on the nanoislands. These nanoislands have stronger interactions with metals than the substrates do, confining metal species within well-defined regions and preventing their migration and agglomeration. This confinement effect is crucial for maintaining the dispersion and activity of metal catalysts, especially at elevated temperatures. Compared with conventional impregnation or deposition-precipitation methods, this approach achieves more precise spatial control over metal deposition and creates strong confinement environments that effectively suppress sintering. This protocol enables the preparation of thermally stable small-sized metal catalysts suitable for fuel processing, exhaust treatment, chemical manufacturing and other uses. The complete workflow, which includes oxide deposition, metal loading and material characterization, typically requires about 4 d, and catalytic testing requires an additional 5–20 h depending on the system. The protocol can be implemented in laboratories equipped with standard wet-chemistry facilities and with prior experience in synthesizing inorganic nanomaterials.</p>