It's a lot simpler to create the Guestlist form before you begin setting up your Facebook app. Please see this video or this article for instructions on creating a Guestlist form.
1. Log into your MyGuestlist account
2. Click on the Social Tab
3. Click on Facebook Applications
4. Click New Facebook App in the right most corner of the window.
5. Choose the Fan Page(s) that you would like to add your new Facebook Application.
Custom applications can not be installed to profiles or groups.
6. Enter a name for your app (only you can see this)
7. Select which type of app you would like.
8. Enter a label for your app, this is what the patron will see on your Fan page.
9. Select to use an existing form
10. Select from one of your previously created guestlist forms.
11. Click Create My App to apply it to your fan page with the details provided.
To style your app, please see this article.