Caulking large gaps involves preparing the surface, including cleaning and ensuring a snug fit. Use a backer rod to fill deep gaps, and select a caulk suitable for the specific application. Apply the caulk with a caulking gun, expertly filling the gap. Smooth the caulk using a smoothing tool to ensure a neat and watertight seal. Allow sufficient time for the caulk to cure before painting or exposing it to moisture.