How to perform the "Snap to Grid" effect?

Hi, I am currently building a chinese puzzle game called tangram. I am wondering is there any method that I could do to make the movie clips to perform the “snap to grid” effect? i.e. the tangram puzzles being snapped to the pattern. I have tried to draw my own and used the “cacheAsBitmap” function to implement it, however it doesn’t seems to be working. I have just written the code for building the grid, but what should I do next to make the snapping action? Is there any suggestions or tutorials to this problem? Thank you very much.