The easiest way to do it is to bypass the computer altogether and get yourself a DVD recorder. Then you just hook up the two units and do a machine to machine dubb
If you don't want to go that route (mind you, DVD recorders are getting pretty cheap) you will have to find a way to bring your analog (VHS) sources into the digital world
One way: A DV converter like the Canopus. Will allow you to do a Capture Now from your VCR thru the Canopus into the puter. You could also use a DV cam as a pass thru device; analog from the VCR to the cam, Firewire from the cam to the puter.
Once you have the tapes in the puter you have to get them into MPEG2 (DVD) format. You can do that by Exporting as Quicktime Movie and dropping the resulting file into iDVD and making a DVD there
This is more steps and more time than using a DVD Recorder but it gives you mo options; like cleaning up the analog signal a little (the newest Canopus converters have TBCs) and using FC/iDVD to create chapters, menus, etc