Assume you have a supported device (e.g., Samsung Galaxy S III i9300). Here’s the process for a quality result:

If you rush into the without prep, you will end up with a bricked device. Follow this checklist:

If you want the original experience for a Google Nexus device or a specific Samsung/Sony phone, you can find original images at:

If you're looking to download Android 4.1.2, you'll need to find a reliable source for the update. Here are a few options: