For this you need 3 things.
- Bucket with Water
Have Hoagie grab the bucket from the kitchen behind the swinging door in the Main Hall then use the bucket on the Water Tap to fill it. In the room on the right from here there is a cupboard open it and collect the brush.
Now we just need soap head upstairs with Hoagie and enter the first door on the right (Hoagies Left). This is George Washington’s room. Have Hoagie “Use” the bed and then “Pull” the cord on the left to summon the maid. This will make her start cleaning the room and you can leave the room. Just outside is the maids cart with a bar of soap. You can now grab the soap!
Now we have everything we need so head outside and go to the carriage. Use the Soap on the Bucket of Water to combine the two and then use the Brush on the Bucket of Water and Hoagie will start to clean the carriage and the achievement will unlock.