To get started with OMNI ROM for I9103, you'll need to get familiar with Git and Repo.
To initialize your local repository using the OMNI trees, use a command like this:
repo init -u git://github.com/karthik050782/android.git -b <branch>
Then to sync up:
repo sync
Then to build:
cd <source-dir>; . build/envsetup.sh; brunch <device_name>
If you need more information or a more detailed guide, check Here Our Official IRC Channel: #omnirom - USERS , #omni - DEVS