it all starts with mutations. The instructions to build a protein sit in the DNA, and DNA can mutate. Sometimes, a mutation can change the blueprint to build a protein and when the cell reads that blueprint, the protein that gets produced differs – often only slightly – from the original protein.
What happens then? Well, that depends: often you can change the building instructions slightly and the protein still works. In that case, the mutation can get passed on to the next generation and so the protein changes from generation to generation through evolution. But sometimes, the mutations hits something really critical to how the protein works, like taking the steering wheel out of a car. If that happens, the organism with the new mutation might be very sick or even die and the mutation will not get passed on to the next generation.
Does that answer your question or did I miss the point?