Move the .zip file to your SD card; make sure you have 80% battery life. Turn the phone off. While holding both volume buttons, press the power button. Press the volume down button until you highlight the boot into recovery option, then press the volume up button to select it. When you see a green Android with a caution sign, press both volume buttons simultaneously to show the options. Press the volume down button to highlight the install update.zip from sd card option, then press the power button to select it. Highlight the SD card storage, press power to select it, then highlight the update.zip, select it with the power button, confirm that you want to install it, and wait for the process to finish.
I haven't touched my recovery in a while so I may have skipped a step or two but I gave you the basics of how it works. If you've seen anything like it you should be good to go as soon as you boot in recovery.