The main way to design and craft UI icons for products is to start by creating a consistent grid and stroke width that you want to use. This will help you maintain consistency throughout your project.
Then I'd largely suggest taking account of all the icons you think you currently need. Ask yourself these questions: What scale will they need to be at? Who will be looking at these icons? How does it fit within the larger style guide of the company you are working at?
These questions will help guide your work.
The next most important thing to understand are keylines which will guide not just the size of the icon but the amount of visual space they take up.