You don't need to do much, the /agent command is the most useful, and it walks you through it. The main thing though is to give the agent something to work with before you create it. That's why I go through the steps of letting Claude analyze different components and document the design/architecture.

The major benefit of agents is that it keeps context clean for the main job. So the agent might have a huge context working through some specific code, but the main process can do something to the effect of "Hey UI library agent, where do I need to put code to change the color of widget xyz", then the agent does all the thinking and can reply with "that's in file 123.js, line 200". The cleaner you keep the main context, the better it works.