After you got
catch the cat
*** Spoiler - click to reveal ***with the mouse toy you get in the past. To get the toy, have Hoagie go upstairs into the attic, and sit on the bed closer to you. Then sit on the bed further away. Then use the squeaky mattress with the other bed. After the cut-scene sit on the far away bed with the squeaky mattress and immediately pick up the mouse toy.
Flush the mouse toy to Laverne.
Have her use the mouse toy with the cat on the roof.
When you got the cat, get Bernard to flush the fork you found in
to Laverne.
Have Laverne use the fork on the cat to get the achievement.