Move "AltServer.app" to your Applications folder and launch it (it will appear as an icon in the menu bar).
Connect your iPhone or iPad to your computer and make sure it's unlocked and your device is trusted.
Mojave users need to open iTunes and enable "iTunes Wi-Fi sync" for your iPhone/iPad.
Catalina users need to open Finder and enable "Show this iPhone when on WiFi" for your iPhone/iPad.
Click the AltServer icon in the Mac menu bar and click "Install AltStore" and select your iPhone/iPad.
Enter your Apple ID email and password. (BOTH are case sensitive)
Your Apple ID and password are sent to Apple, and nowhere else.
AltServer will ask you to install a Mail plug-in, follow the instructions to continue. This is only required the first time.
Wait a few seconds, then AltStore will be installed to your iPhone/iPad.
Make sure you're on the same connection (same router, in most cases) as the computer running AltServer. If you're not connected to the same network, you can connect your device via cable.
Trust AltStore application: On your device, go to Settings > General > Device Management. Choose your Apple ID and select Trust.
Launch AltStore on your iPhone/iPad.
Go to My Apps and tap the + icon in the top left corner.
Open the .IPA you downloaded from the Prerequisites section.
Make sure you're on the same connection (same router, in most cases) as the computer running AltServer. If you're not connected to the same network, you can connect your device via cable.
Trust AltStore application: On your device, go to Settings > General > Device Management. Choose your Apple ID and select Trust.
Launch AltStore on your iPhone/iPad.
Go to My Apps and tap the + icon in the top left corner.
Open the .IPA you downloaded from the Prerequisites section.