This guide requires a PC.
The following instructions apply identically for PPSSPP and PPSSPP Gold.
First, install the PPSSPP app. (There will be a link to a separate guide here soon).
Connect your Android device (phone, tablet, etc) to your PC using a USB cable. Likely, the one you charge the device with will work just fine.
On your device, pull down the menu and click the circled arrow:
Then click the circled item, "Tap for more options".
Next, you'll be at this screen, where you can now enable USB file sharing:
On your PC, you'll now be able to see the device's storage in Explorer:
You can now create a new folder to store your ISOs in, if you haven't already (it's recommended that it's separate from the "PSP" folder where system files and save games go, but it does work to use the same folder). A quick way is by pressing Ctrl+Shift+N and typing the name of your new folder.
Finally, after starting PPSSPP on your device, you'll be able to either directly browse to it (Android 10 or older, or old installs), or you'll be able to click Browse... and select it.