It hasn't been possible to create an FBML app since March 2011, and the complete removal is on the roadmap for June 6th and was flagged back in September 2011
There's no 'transition' guide so to speak, just reimplement your app's content in an iframe, and if you need interactive content, follow the documentation for the currently supported APIs
If it's just static content you want to display on a page tab, etc, here's some 3rd party apps that you could use instead of reimplementing your own app:
- WildFire iFrame app
- Static iFrame Tab
- Static HTML Plus
- Involver iFrame app
- Tabpress