Multi-piece, flexible, sill pan assemblies and methods of installation are disclosed. Method of installation of a sill pan assembly for pocket door systems such as single- and double-door versions as well as multi-panel pocket doors. The sill pan assembly has a first flexible end dam member, a second flexible end dam member, a flexible outside corner member, and a flexible insert. Methods of installation of a sill pan assembly for a double-pocket door. The sill pan assembly having a first flexible end dam member, a second flexible end dam member, a first flexible outside corner member, a second flexible outside corner member, and a flexible insert.