A good step by step example of backpropagation (gradient descent in sigmoidal networks)
https://mattmazur.com/2015/03/17/a-step-by-step-backpropagation-example/
A more in-depth treatment of backpropagation
https://page.mi.fu-berlin.de/rojas/neural/chapter/K7.pdf
https://drive.google.com/file/d/1Mj4sY1uE2Mk1_lcJZnJudVzI1oBkZ_b3/view