You want to do Ad Hoc distribution. I will assume that you are enrolled as an Apple Developer in the Paid iOS program, you have downloaded and set up a developer certificate, and you have already registered the test device in the Apple Developer Center.
- Create a Provisioning Profile for distributing Ad Hoc.
- Set the Code Signing in your app to use this profile.
- Create an Archive of your app for Ad Hoc with this new profile.
- Take the Archive file and upload it to an ftp server. Make note of this location.
- Create a plist file like shown here, and use the location you noted in step 4.
- Upload the plist file to the ftp.
- Create a web page with a link to the plist file. It should trigger the download and will be successful on devices that are included in the provisioning profile you made in step 1.
Note: TestFlight is a popular alternative which has the added benefits of analytics, etc.