1. Have a blackberry with internet, a usb cable, and a laptop (this one has windows 7 but XP works fine too).
2. Download blackberry desktop software. You need it so your PC can recognize the BB as a modem (in theory you just need the Blackberry device manager).
3. Connect the BB to the PC via the usb cable. The Blackberry device manager should appear in the taskbar.
4. Type "modem" in the windows search that appears when you click the windows key. Select "Phone and Modem" from the search results.
5. Choose a location if you haven't done so before - this doesn't matter.
6. Click the "modems" tab. The blackberry is one of the standard modems. Click properties.
7. In the new window, click on the "change settings" button so that you can modify parameters.
8. Choose the "diagnostics" tab, and select the query modem button. It should respond with proof that this is the blackberry device.
9. In the advanced tab, enter the following string depending on your carrier:
read this post if you want to find out more carriers
10. Setup a new dial up connection. For both of the carriers above, the number to dial is *99# and username and password are left blank.
That is it. If you run into problems, follow the tips on the post above, but these steps worked on a fresh windows 7 install - simple enough.