See all those cans and bottles and car repair stuff on the shelves? Actually, there's nothing on the shelves but a few soft-edged strokes. In fact, there aren't really any shelves. Take a closer look:
| Unchained, detail
The context tells you what you might expect to see, and you do at least half the work of putting it there. Keeping the edges soft actually makes it easier for the viewers to meet me halfway, since there is nothing very specific to conflict with their vision of what would be stacked up on that wall. The fact that it's dark in there makes the vagueness appropriate, as does the overall casual approach to describing the scene (e.g. those wobbly window mullions).
Using one of the photos from class, or an image of your own that includes a passage or two of layered soft-edged forms, practice the wet-on-wet part till you feel confident, and then make a version of the whole image.
When you're done, begin a written list of some good questions to keep in mind when you need to control the spread of the paint. Bring everything, including practice sheets, to class.