First things first, we need to change the date on your iOS device to work around Apple's restrictions.
Open the 'Settings' app, go to 'General', and choose 'Date & Time'. Uncheck 'Set Automatically' and choose any date before February 19th, 2014.
Go to the GBA4iOS website on the iOS device you want to use and tap 'Download GBA4iOS 2.0'.
Click 'Install' on the pop-up. The app will be downloaded and appear on your Home screen like any other.
The first time you load the emulator, it will ask you if you're sure you want to open the app. Choose 'Continue' to proceed.
Return to the 'Settings' app and change the date back to today.
If the app ever gives you trouble, just change the date to something before 19/02/2014, load the app again, then change your date back.
Back in the app, hit the '+' symbol in the top-right-hand corner.
You'll be taken to a website where you can download ROMs. From here, you can find a game you want to play and download it. You also get to name the game if you're not happy with the ROM title.
When the game has finished been downloaded, hit the name of the game in your ROM list to start playing it.
If you'd prefer to go the legal route and use ROMs that you've backed up from your physical cartridges, start by plugging your iOS device into your computer and loading up iTunes.
Click on your gadget under 'Devices' in the left-hand column, and then click on the 'Apps' tab.
At the bottom of this screen is a section called 'File Sharing'. Find GBA4iOS and then simply drag your .GBA or .GBC files onto the table on the right. Then, hit 'Sync' and unplug your iOS device.
You can also sync with your Dropbox account through the app's settings if you keep your ROM collection in the cloud.
If you own one of those fancy iOS 7-compatible controllers, just plug it in (or sync it up over Bluetooth) and it will work.
You can change the buttons by going to the ROM list, hitting the gear icon, and choosing 'Configure Buttons'. The images in the background represent the buttons on your pad, while the pop-up balloons are the Game Boy Advance buttons.