Quiet simple,
1st you'll need the ( Haunted Mask ), to get this go into your house, go into the upstairs bathroom and open up the sink cupboard, and dunk random items down the chute, once that's done, go into the Laundry room, and the mask should be sitting on the dirty washing pile, the push through the game (not writing all that out here lol), once you're in the book store. put on the mask, then interact with the doll in the room once that's done, Slappy will jump out, Go through his dialog once you get the option to either run or fight, just fight him and the achievement should unlock (providing you've done all the requirements to finish the game)