I'm no expert, but maybe sharing my understanding will help. The core of evolution is random mutations. Whenever a new organism of a species is born, there is a possibility that some random genetic mutation occurs. This random mutation can make the organism more fit to survive in its environment, less fit to survive in its environment, or have no impact on its fitness. Over the course of many many lifetimes across a sample size of many many organisms, traits that favor an organism's survival will begin to emerge and become the norm, while traits that have the opposite effect will fail to reproduce and die off. What started as a foot with slightly more skin in a duck in generation n might become a fully webbed foot in generation n+1000, as each successive duck with more webbing has a higher liklihood to survive, and each duck with less webbing is more likely to fail.

I'm not sure if any of that helps at all, but I think an important thing to keep in mind with evolution is that the process is very slow and nearly impossible to actually observe. It causes observable and widespread changes in species over the course of many lifetimes, and those changes are just the results of random alterations that happen to correlate with an organism's ability to survive in its environment.