The process you describe as applying shadows based on how you think the shadows would be cast in the end is correct. This isn't working from "imagination" in that it still requires you to understand the nature of the forms that are present and casting those shadows (which is explained in this section of reminders).

If you find that they come out looking like lines, then it's likely that you're not drawing them as shapes first - which means outlining the shadow shape itself first (so you can purposely design the shadow shape based on the relationship between the form casting it and the surface receiving it), then filling it in. You'll see this demonstrated in this video from Lesson 0, specifically in the section discussing brush pens. If you try to "paint" the shadows on stroke by stroke, they're more likely to read as lines.