STEP 1 JAIL BREAK YOUR iPHONE
You've got a number of jailbreaking options, including PwnageTool, Redsn0w, and Blackra1n. Pick one that works on your platform, download it, and walk through the jailbreak process. (I'm not going to detail the jailbreak here since the latest jailbreaking methods often change a little, but at this point, jailbreaking an iPhone is a relatively simple process.)
STEP 2 INSTALL BOOTLACE IN CYDIA
In order to do this, you'll need to launch Cydia from the homescreen, tap on the Manage button, select Sources, then tap Edit, then Add. The repository you'll need to add is repo.neonkoala.co.uk. Tap Add Source, let Cydia work its magic, and then tap on your newly added repository and install Bootlace
STEP 3 RUN BOOTLACE AND PATCH THE KERNEL
Leave Cydia, and launch Bootlace. If Bootlace isn't on your homescreen after you leave Cydia, restart your iPhone and it should be there. Then just launch Bootlace, and let it download and patch the kernel. When it's done, tap on Reboot and wait for your phone to reboot.
STEP 4 INSTALL OpeniBoot
Now launch Bootlace again, tap the OpeniBoot button, and tap Install, then Continue. OpeniBoot will download and install.
STEP 5 INSTALL iDroid
Once OpeniBoot is installed, tap the iDroid button, tap Install, OK, and then wait. iDroid—which is essentially the Android OS customized for your iPhone—will download and install. This will take a while, so be patient, and if your battery is low, you may want to plug in your phone before you start.
STEP 6 BOOT AND PLAY WITH ANDROID
Android On iOS