If you have found a community-created racing mod or ISO, the installation process follows the standard procedure for the PPSSPP Emulator or a homebrew-enabled PSP console. For Android/PC (PPSSPP)
However, the "Forza Horizon PSP ISO" search typically refers to —primarily total conversions of older PSP racing games like Test Drive Unlimited or Gran Turismo —that attempt to mimic the Forza Horizon interface and car list. The Reality of Forza Horizon on PSP forza horizon psp iso install
: Your PSP must be running CFW (like PRO or LME). If you have found a community-created racing mod