If your VCR has a SCART output then just plug the SCART lead into the AUX socket on the TiVo and 'change channel' to a channel that would be received from your STB.
If you are using RF (aerial only) for your programme source, or your VCR doesn't have a SCART output, then plug the RF output of the VCR into the RF-IN on the TiVo and try all the aerial channels (including the ones you don't normally receive) until you find the channel the VCR is outputting on.
6023, Lifetime, 240gb, Sky, CC, Mode 0, 100 SPs/AWLs
6022, Lifetime, 250gb, Freeview, CC, Mode 0, 90 SPs/AWLs
601E, my 'test' machine!