If you already have an ileostomy DO NOT do the prep! 24 hours of clear liquids will do the trick. Prep can severely dehydrate us, something which most doctors/nurses do not take into account - instead they just hand out the usual 'how to do prep before surgery' sheet that they hand out to everyone not realising it can be dangerous for ileostomates.
You can try ringing the person who sent out the instructions, but usually, the most you will get is 'oh, only drink half of it then' (after they've run around trying to find out what the heck an ileostomate is and even then they're not told the full facts of it).