How difficult it is, is a matter of perspective. If you haven't learnt the basics of developing a B4i app, it will seem impossible, so don't just jump in a try to port the whole thing at once. Take it in steps (as you should have done when you developed the original app) which should be easier now you fully understand how your app works.
The process will be different depending on what type of app you are porting but I would start by developing a Gui for the main screen so you can see it working as you'd expect, then add the non Gui functionality (which should be at least 90% cut and paste). Then do the same for the remaining screens or views.
Like anything else, the more you do it, the easier it will become.