1. To start off, get to Act II
2. Talk to Granny Weatherwax about elves and she'll show you the location of the Stone Circle.
3. Go into the Circle, then come back out again.
4. Go back in again. Repeat about ten times.
5. Instead of going to the Elven Kingdom, you'll end up in Rincewind's bedroom from the beginning of Discworld 1!
6. You will then meet the Discworld 1 version of Rincewind.