I realize you said you don't know how to navigate Xcode, but I'd say your best option is to use that.
1. Download Xcode
2. Open Xcode
3. Pick "Create a new Xcode Project" from the "Welcome to Xcode" window if it automatically appears, or go to File > New > Project in the menubar if that window doesn't appear.
4. When it asks you to choose a template, choose "OS X" on the left, then "Cocoa Application" in the main panel. Click next.
5. I assume you can figure out a Product, Organization, and Company name on your own. For "Class Prefix", just put down an abbreviation for your product name. IE, if your project is called "Battery Status", you might use "BS". If your project is called "Untitled Space Shooter", you might use "USS". If your project is called "My Project", you might use "MY". Category seems pretty self explanatory. If your application should create documents, check off the box and say what it'll use as a document extension. Core Data and Spotlight... eh, it really doesn't matter. Hit next when you're done.
6. Save it wherever you want to save such things.
7. The left panel of the main window shows the files in your project. Files ending in .xib are your interface files. Open a .xib file by clicking on it.
8. The middle panel should now be split in two (if it didn't click on the button in the lower left corner of the middle panel to expand the split.) On the smaller left side of the divide, there's a hierarchical list of objects in the file. Click on the Window or Document object on the left panel if you can't already see it on the larger right side and it'll jump into focus.
9. The rightmost panel has two tab bars - one at the top of the panel and one near the bottom of the panel. Since you're entirely focused on UI, there's just 3 tabs you need to know about. If you mouse over them, the tooltip will appear and tell you the name of the tab.
i - On the bottom tab bar, the only tab you're interested in is the "Object Library tab." It's icon is a cube. Click on that and underneath it you'll get a list or grid of UI elements that you can click and drag into the main panel. You can search the list with the search bar underneath it. You can toggle between a grid and a list by clicking on the button next to the search bar.
ii - On the top tab bar, a tab you'll be interested in is the "Attributes Inspector". It's an icon of a pointed slider... it kind of resembles a badge and is near the middle of the bar. Click on it and you'll be able to change attributes of the item you have selected.
iii - Again, on the top tab bar, another tab you'll be interested in is the "Size Inspector". It's an icon of a ruler. Click on it and you'll be able to change the size and position of the item you have selected.
Oh, also, at the bottom of the main panel there's a few buttons. You might be interested in the second, the "Pin" button, which lets you set rules about how elements reposition themselves when the size of their container changes.
10. If you want to test your interface, go into the menu bar and pick Editor > Simulate Document.
A final note... if you want to add more .xib files to your project, go to File > New > File, pick the OS X > User Interface category, and pick the type of stuff you'll be putting in the new .xib file. There's really no need to add extra .xib files, you can use just and stick multiple windows in it, but if you want to be more organized, that's how you do that.
That should be everything you need to know to make the interface for a standard, Cocoa OS X application. Further, when you want someone (possibly yourself) to write code for it, they can just take the project your started and wire it up to some code to make it work.
