All of the other answers are either out of date or too much work where it doesn't need to be. Upload your .IPA file to diawi.com then either scan the QR-Code and install, or email the link to the device you want to install the app to, or type the shortened URL into your Safari browser and install that way.
I needed to get an app installed into an older iOS device today and this method took me less than 2 minutes to complete start to finish.
https://www.diawi.com/