The easiest method is to use an emulator that is available directly on the App Store. For iPhone users, look for real PS2 emulator options like Play! or other clearly listed PS2 emulator apps, then check the reviews, update history, and supported features before installing.
Play! is one of the main real PS2 emulator projects with iOS support, but availability can depend on your region and current App Store listing. If it is not available in your App Store, do not jump to random IPA download pages. Use the official Play! download page or trusted sources only.
If an app promises perfect PS2 performance on every iPhone, be careful. PS2 emulation is still heavy, and many games will not run smoothly on iOS.