1. Download and install the Google Android SDK. The tool you need is adb.exe. Just typing adb in a command shell will display all the options. I'm not going to cover adb in detail, it's the SDK tool you use to install applications and interface with the device.
2. Download the USB driver for Windows. You need it for adb to inteface with an actual device using the included USB cable. Install it later after connecting the G1 to your computer using the USB cable.
3. Go to Settings/SD card & phone storage and disable Use for USB storage. You can enable it again later after you installed your third-party application.
4. Go to Settings/Application settings and enable Unknown sources.
5. Connect the G1 to your computer using the USB cable and install the driver you downloaded in step 2. After installing the driver you should see ADB Inteface in Windows Device Manager.
6. If you made it this far, download the APK file to a local folder on your computer, something like C:\MyAPKs will work fine and install it using ADB. The command would be adb install c:\myapks\