The best way to learn it is to just do stuff and see what it does.
Things like Prefixes and Custom Fields are core systems, so Showcase Item Prefixes are managed just like Thread Prefixes are, so if you know how to managed thread prefixes, Showcase Item Prefixes are exactly the same as far as creating them, associating them with a Showcase Category etc etc etc. Same with Custom Fields, Showcase uses the Core XF Custom Fields System, so anything you can do with Custom User Fields and Custom Thread Fields, you can do with Custom Item Fields and Custom Review Fields in Showcase
Options for Showcase are handled via the Core XF Options System.
Showcase Categories use the Core XF Category System as a base, however, Showcase has A LOT MORE per category settings/options than say the XFRM or XFMG that also use the Core XF Categories System.
Showcase is heavily permissions based, so
one of the first things you will want to do is set all of the Showcase Permissions (user group permissions). There are A LOT of them as Showcase is packed with features and needs a lot of permissions. You might want to consider just setting permissions for your admin group so that you can learn it with out your forum members seeing it
The best way to learn is to create some categories and configure those categories differently so that you can see how per category options work.
You should also create several (dozens) Showcase Items, so that you can see how things look with multiple pieces of content (its hard to understand what Grid View looks like if you only have 1 test item, you need several (which is why I recommend creating at least a dozen). And don't just do "asdf asdf asdf asdfasdf" for titles and message content, that doesn't give you any learning experience and it looks like crap. Use
Lorem Ipsum - All the facts - Lipsum generator for content needs. Upload attachments, so that you can see how the cover image system works.
Learning by doing is by far the BEST approach and I am always an inbox away for any questions that you might have. Create some content, change various options and category settings to see how options/settings work.
If you are familiar with XF Nodes, Thread Prefixes, Thread Fields, then you will have no problem understanding SC Categories, Item Prefixes and Item Fields as they are managed in the same manner (for standardization).
Also, Showcase comes packed with several Widget Definitions that allow you to create tons of different types of Widgets that can be displayed anywhere that you want (either via built in widget positions or manually adding a widget where ever you want to).
Hope that helps