A post sleeve installation assembly includes: an elastomeric sleeve core shaped to form a post sleeve when positioned in uncured concrete, a removable stiffener inside the core, an aperture extending lengthwise in the stiffener, a locking element positioned within the core, and an assembly plate coupled to the stiffener. The assembly plate clamps to a prefabricated sleeve element, with the core and stiffener positioned within a post cavity of the sleeve element. an installation stake is positioned in the aperture and prevented from sliding upwards by the locking element. A release point on the assembly plate permits an operator to release the locking element for removal of the stake. To install a post sleeve, the operator positions the installation assembly, clamped to a prefabricated sleeve element, in a post hole with the stake resting on the bottom, then fills the hole with concrete, manipulating the stake to position the assembly.