First, you have to root your phone.
You have to follow the directions and use the software (for PC) from unrevoked.com
And while your phone is rooting, you can download Fusion ROM and the G-apps, and when that's done, put your phone in disk drive mode, or go into recovery and select mounts and storage, then mount USB or something like that- either method works.
Cody that zip file of the ROM you downloaded into your SD card (make sure you don't copy it into any folders on the SD card!).
Now turn your phone off.
Press and hold the power button and the volume down button for a couple of seconds and you should be on a white screen with skateboarding Androids.
use the volume rocker to scroll to "recovery". If recovery isn't in that list, select "bootloader" then in the new screen that comes up, select "recovery".
Now the phone should reboot into a black screen with green lettering- ClockworkMod Recovery.
Use the optical trackpad to scroll down to wipe data/factory reset. Hit the optical button and when that's done, select "wipe cache partition".
When that's done,
Use the optical "trackcircle" to scroll down to "install zip from SD card".
Press the trackcircle button to select. Now scroll to and select "choose zip from sd card".
Scroll down to something like "fusion inc ota" should be at the very bottom of the list.
Select it and now you're installing Fusion ROM! Exciting, right?
After that's done a menu will come up, select "install zip from Sd card", then "choose zip from sd card", then scroll to "Fusion-GAPPS", and install that.
Once that's done, select "reboot system now".
Done!