To register yourself in the world leader boards you must first be connected to Xbox Live. Make sure you're on the Stage Select map menu so that you can access Ranking Mode by pressing . The map should turn dark red. Now play all 7 Stages start to finish under Ranked Mode on Normal difficulty or higher. Upon completion of a stage the game will ask you if you want to save your score. Select yes and it will be registered on the world leaderboards. Now do this for all other stages for the achievement.
Some notes on Ranking Mode:
There is no Easy difficulty (setting will change to Normal if Story Mode is on Easy)
If you die, mess up on a QTE, or a mini game you'll have to start all over
You can still find lollipop wrappers and zombies(for album) in Ranking
There is no Chop2Shop.Zom, a zombie chicken enemy will take its place
Medals collected do NOT carry over to your Story Mode
Ranking Mode is also recommended for working toward the zombie hunter(beat Dad's score) achievements as there are bonus requirements for each stage section that help you get more medals that aren't present in the Story Mode version.
Note: To check which stages you've done go to Juliet's House on the stage select screen and choose the leaderboards. From there you'll see three options for Difficulty, Level and Mode. Change it to ranking and you can look to see if you're scores are posted for Ranked.