You'll need to do the following:
- Build the app using the distribution cert of the client
- Archive the app using XCode (it's in the same menu as the build
options)
- When the archive is done it will open the Archive
Organizer
- Select the newly created archive and export it. The
first option in exporting is the 'Save for iOS App Store
Deployment'.
- This will create a directory on your mac that contains the bits
and pieces your client will need to upload it.
- Zip that directory and transfer to your client. They should be able
to use the web-interface in iTunes connect to upload the app and
complete the app submission process.