As you progress through the game, you will see piles of Legos that appear to be hopping around. Walk up to this pile and press and hold to build an object. Repeat 249 more times for the achievement.
If you don't get this achievement by the end of your 2 playthroughs, you can boost it quickly on Movie 3 Stage 4 (trouble in the sky). When you get to the beach area, about halfway through you will need to build a wooden platform that you then use to get over the next wall. It will collapse under your weight almost immediately and can be rebuilt an infinite number of times. Use a female character and she can just jump right on top of it instead of having to get on top of the building and swing over.