Here's how you get into the Stonecutters Guild and Mr. Burns' fireplace.

First, go under Bart's bed, attached to the top is a note, grab it. Go to the school, go to Bart's locker (one on the left) and look for a paper that you can grab from the side of the door. Go to the retirement center and look in the bottom drawer. There should be a ring. Now go to STONECUTTERS and Homer will be singing. Crawl under the shelf and look in a box. There's the key to Mr. Burn's fireplace.