Onset Theory (previously known as Strain Invariant Failure Theory) is a physics-based composite failure criterion attempting to predict failure at the level of constituents rather than for a homogenized ply. Applying Onset Theory correctly requires a number of steps. As a starting point for other researchers, a consistent approach is developed by resolving contradictions found in literature. This includes the micromechanical FEA process used to determine the individual fiber and matrix strains, such as correct choice of representative volume elements (RVEs), the use of transformed RVEs, boundary conditions, and the location of data extraction points. Analytical expressions are provided to determine the full state of strain in a ply, including in-plane and out-of-plane Poisson’s strains and thermally induced mechanical strains. Literature definitions of the critical invariants are discussed, and a trend in critical distortional invariants is observed. Assumptions and limitations of the theory are identified. Finally, a failure envelope is compared to World Wide Failure Exercise test data.