How do I design a system from scratch when it overwhelms me?

You can follow a design spec, but building a system from raw requirements overwhelms you, because now there is no template, just a blank page and a hundred choices. That jump from following a design to creating one is a big step, and feeling lost at the start of it is completely normal. Designing from scratch feels impossible because you are trying to see the finished system before you have begun. That is not how it works. You start with the requirements, sketch a rough shape, and refine it in passes, each one making the design a little more real. The blank page is not asking for a finished answer. It is asking for a first rough draft. With your next design, do not aim for the right answer immediately. Write down what the system must do, sketch one rough approach, then improve it. Show an early version to someone and let their questions sharpen it. A system gets designed the way anything gets built, in layers and drafts, not in a single leap from nothing to done.