From Android Wiki
First off, be advised if this process fails, you have a very pretty looking paperweight and from what I have heard, your dog might die. Your device will not recovery from this if you mess up. That said, HAPPY FLASHING!!!
- Download the engineering bootloader
- Rename it to update.zip and save it to /sdcard on your G1
- Enter the recovery console (like you did when you flashed JF's modified update) by performing the following steps:
- Hold down power and home to enter recovery mode
- At the exclamation point and phone image hit Alt-L look for test-keys on the second line. If missing, you have not installed the modified recovery like you were told to and should stab yourself in the face now.
- Press Alt+S. The system will proceed to verify the image and install it. When done, it will say to reboot by pressing Home + Back.
Confirm your phone has the engineering bootloader by turning the phone completely off, then rebooting holding the power + back button. If you see skateboards, you win.