First be rooted and have Titanium Backup of course. Backup the app you want, then go onto your SD card with some file manager and open the TitaniumBackup folder. There are usually 3 files to a backed up app. The one you need is usually the largest and has the extension .apk.gz. You'll need to extract/decompress it to get the .apk. You can do all of this with ES File Explorer. When it's done decompressing, it creates a folder and within that folder is the .apk.
