The card for Series1s is here. You pop the case (void your warranty), plug it in, and close the case.

For Series2s you just need a USB Ethernet adapter. 9thtee sells one for $17, or there's a whole bunch of other supported ones.

Once you're connected the TiVo gets an IP from your friendly neighborhood DHCP server. You go to Setup -> Phone Setup -> Connection -> Dialing Options and set the Dial Prefix to ,#401 and from then on the TiVo will use the Ethernet for service downloads. This also survives software upgrades, so you don't need to re-do it every time you get updated. If you decide to do more than that there's a whole world of web servers and video extraction/sharing you can use the card for.