To install IPA files, you'll need to download them from a trusted source - don't worry, we won't judge you! Just make sure you're getting them from a reliable website, or you might end up with some malware on your hands. Once you've got your IPA file, it's time to move on to the next step.
You'll need to connect your iPhone to your computer, using a USB cable - yes, the one you always seem to lose! Then, launch iTunes and select your device from the top left menu. It's like a little treasure hunt, and the treasure is your IPA file!