Methods of forming a cooling passage on a turbine component having a component wall with an internal surface and an external surface, are disclosed. An opening is formed passing through the component wall and fluidly connecting the internal and external surfaces. The opening includes a metering section extending from the internal surface to a metering end, and a diffuser area extending from the metering end to the external surface. A preformed cap element is added to close a portion of the diffuser area to form the cooling passage with a diffusion section extending from the metering end to the external surface. The preformed metal cap element includes a projection extending internally of the external surface and into the diffusion area to define an internally facing section of the diffusion section. The cooling passage extends through the component wall and fluidly connects the internal surface and the external surface.