Provably stable boundary treatments for nonlinear problems is an important, but difficult, aspect for initial boundary value problems. This is particularly true at open boundaries such as inflow or outflow regions of the domain. Recently, the nonlinear energy method was used to create provably stable simultaneous approximation terms (SATs) to weakly imposed boundary condition for several nonlinear partial differential equations of interest, e.g., the shallow water equations. The first part of this talk presents how the SATs from the nonlinear energy method can be interpreted in terms of a standard mathematical entropy analysis. This translation is initially done for the one-dimensional Burgers' equation, as its simplicity illustrates the fundamental idea. After this we extend the discussion into two dimensional curvilinear coordinates with the shallow water equations. The second part of this talk discusses how one translates the provably stable open boundary SATs into an equivalent numerical flux formulation that is a function of the external boundary data and internal solution data. The resulting numerical flux is then amenable for use with a range of approximation techniques, e.g., finite volume or discontinuous Galerkin spectral element methods. Implementing the new stable boundary condition strategy into an existing code is as simple as writing a new numerical flux routine. Lastly, we present numerical results to demonstrate and verify the use and properties of these stable boundary treatments in the context of high-order discontinuous Galerkin spectral element methods.