The details for keeping water out of flanged windows vary, depending on whether the job is for new construction or a retrofit, and on whether you are using straight flashing or flexible flashing for the sill.

For new construction using a flexible sill flashing, see this slideshow for best practices and step-by-step details.

For more details on flashing windows and doors, including flashing replacement windows and arch-top windows, visit the JLC Field Guide.