AltStore:
Download AltStore from and install AltServer on your computer. After that, install AltStore to your phone through AltServer.
Download the YGO Omega IPA from here: -unite/omega-releases/releases/download/Latest/ios.ipa It will be automatically saved to your device in downloads, or you can move it to wherever you prefer.
Open AltStore on your phone and press the + button under My Apps. Find the YGO Omega IPA (named ios.ipa) in your file system and click on it. AltStore should do the rest.
If you have any problems with AltStore, please refer to the official FAQ found here: AltStore Docs - AltStore. This is the BEST place to find information for any issues you may encounter.
For Windows users: be sure to install iTunes and iCloud from the apple website and NOT the Microsoft store.