Like pretty much anything on Cydia, you just search and install. Perform a search for RetinaPad, pay the $US3 fee, and you're off to the races. Once RetinaPad is installed you'll need to restart Springboard and launch any iPhone app you want to enable. When you launch it for the first time after installation, RetinaPad will ask you if you want to let it run regularly or use graphics designed for the retina display. If you choose the retina display, you'll need to close the app and then quit it via the multitasking bar. When you launch the app again, it will use any retina graphics available. This may not be much more than standard user interface elements (e.g. the keyboard), but it makes a huge difference with any apps that support the retina display but not the iPad (like is the case with many games). The $US3 price tag won't be worth it for everyone, but if you've got a ton of apps that look bad on the big iPad screen, RetinaPad can improve your experience in a meaningful way.
RetinaPad is available on Cydia for $US3 [@ryanwagner via @howtogeek]