Last year we dropped off a Stuffed Minnie Mouse wearing a cruise director outfit and all the characaters signed it for my daughter. Note: Only the Mickey Mouse type characters signed it...Mickey, Minnie, Pluto, Goofy, Daisy and Donald Duck, Chip & Dale, and I think a couple more.
This year, I dropped off my daughter's princess pillowcase from home and all the princessess that were on the pillow signed it next to their picture..Belle, Cinderella, Aurora, Snow White. The other princesses on board didn't sign...I guess since their picture wasn't on it?
Also this year, we dropped of a 2005 5oth anniversary Mickey Mouse for my son and Mickey and Minnie signed it. It will be a nice keepsake since he was born in 2005.
From what I understand you have to be discrete. You drop it off towards the beginning of the cruise and the last night of the cruise you pick it up at guest services or sometimes they deliver it to your cabin.
We forgot sharpies but they signed in different colors anyway.