These are shown by an eye icon (the same as the achievement picture) on your map and are started by talking to Neji. There are a total of nine missions and in each one you must find a sticker.
As soon as you pick up the mission use your Byakugan jutsu. This will make a circle appear at the bottom of your screen leading you to the sticker. Make sure that the green part of the circle is facing upwards and you're headed the right way. Find the sticker within the time limit and return to Neji.
You can check your progress at any time by holding then looking at Challenges.