If you're just looking to make it work, the links @Lodder provided will help massively when dealing with deprecated code. A bunch of 3.x code was backported while working on 2.5.5, so if you're only supporting that version or later, you could theoretically get a component working on both 2.5 and the 3.x releases with relative ease. What isn't documented is the layout changes that came with the switch to Bootstrap; you'll need to use the core components as a model for that.
If you're looking to add support for 3.x features (like tags or content history), you'll need to do some more work. These links will help you integrate support for those features: