Don't you use a computer? That's fine.
I would make a copy and then make a copy of the copy. And keep going back and forth. The quality should degrade each time you make a copy of a copy.
Also, record over a really old vhs (one that has something on it that you don't mind losing), if possible, that will also bring the quality down.
Not sure why you'd want to do that, though. And I also kinda agree with chilipie...