We present Heinz' approach to Brouwer degree in a simpler, shorter and better motivated way. We link it to Kronecker index, use the language of differential forms at an elementary level, and prove the homotopy invariance using an unpublished result of Tartar.
Mawhin, J. (2004). A simple approach to Brouwer degree based on differential forms. Advanced Nonlinear Studies, 4(4), 535-548. https://hdl.handle.net/2078.5/44812 (Original work published 2004)