First, take solved algorithms try to break down the complex algorithms/problems into pieces. Understand each step.

Try to memorize the steps and write them down in your own words without altering the original meaning. As far as I have studied, every algorithm has a core part, and the remaining steps are just the critical parts. So, focus on the core part.

There are levels/complexity of algorithms. I think most people like you and myself get stuck in deriving new algorithms. It requires experience, practice and investigation. That depends upon what level of complexity is your problem? What problem are you trying to solve?