Be sure to start with the manual article on IAP
From there, it contains specific tutorials/links to the various platforms (each platform has a different interface and is maintained by a different company, as such, its best to rely on their documentation to be up to date on how to sett items in their interface).
Once you have correctly set the item on the backend of the platform, enter the ID for this item in the IAP plugin for Construct 2 and use the conditions/actions mentioned in the manual article.
You can indeed also search for the IAP keyword
in the tutorials section