NOTE: This article assumes that you have already turned your PSP games into ISO/CSO files, see the separate article on that. Additionally, 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 savegames go). A quick way is by pressing Ctrl+Shift+N and typing the name of your new folder.