You can't.

Not only does T-mobile lock down the sidekicks by hobbling their OSes considerably over the original HipTop OS, but the danger back end will completely prevent any tethering from working.
The back end servers I'm referring to are proxy servers that resize images to fit on the low res sidekick screens, and preprocess much of the web pages so the CPU in the SK can handle them.

You can find much info in the discussions on hiptop.com forums. Or at least you used to... I just went there and it looks nothing like the old site. In fact I just checked skdr.org and it is GONE (SideKick Developers' Resource). I was going to mention the latter so you could see all of the cool apps that T-mobile won't let you run, because they haven't decided to put them in their "catalog" yet.

I've had sidekicks for close to 5 years now, and it's always been a love/hate relationship.
I love the form-factor and the feel of the device, the ease of typing, etc... but the combination of T-mobile's hobbling and Danger's slow progress are infuriating (some of the things Danger has been promising are in the next release have been in that state for the entire time I've had the devices).