Digital exhibit-building is digital storytelling.
Through the medium of Omeka, you'll be preparing an illustrated historical essay using primary source materials largely from Butler's collections. You need to take care that you're telling the story of your objects and how they relate to your topic of choice. If they don't, then you've selected the wrong objects.
What's the difference between an exhibit and a gallery? This blog post does a great job outlining the differences. Make sure you're thinking about your audience and their knowledge. Your goal is to educate and help your audience appreciate what it is they're looking at. To that end, an outline for your exhibit from beginning to end will be helpful. What do you want your audience to leave having learned? Then work backward: how do you get them there?