If you didn't download the package yet, start by going here and downloading the Unity IAP package with UDP.
Integrate it on Unity and then follow this guide and then for easiness and speed this guide but first take a careful look at the following list of notes that might help you out dodging some possible errors you may encounter during the process:
- Make sure the product ids for products you create on the catalog are lowercase. This is mandatory for Android but not iOS for example, so Unity doesn't warn you if you have capital letters by mistake. Don't use fullTrajectory use full_trajectory instead.
- Make sure at the end of Unity IAP's IAP Catalog you have the Automatically initialize UnityPurhcasing checkbox enabled. This is crucial for the process to work with Codeless IAP for instance and sometimes it does not start enabled by default.
- Don't forget to set up a price in USD on the Unity Distribution Portal section
After you're done creating and setting up your products you're ready for Submitting your game ]