Default [Q]How can I port apps?

Hello to xda community!
Recently I found some nice system uis for my device but there was all for ics and I'm running jb 4.1.2. Also, I have searched for guides but there was about "how to port apps to ics from jb " completely different from what I actually want! :banghead: So, I decided to open a new thread and use my best English. I don't want to port the app for me, I want to teach me how to do it myself. I would like to learn how to port from hdpi to mdpi, too.

My device is Sony Ericsson Xperia mini ( st15i) cm aka smultron.

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