In-Game 2D Sprite Building System

I want to delve into the depths of a building a 2D sprite construction system that potential players can use to build objects, So the system will need to be able to allows the user to place different sprites(Different shapes and sizes), there will need to be a vertex snapping system so that the multiple objects placed can be combined into one object. This will then need to be saved as a prefab / object of someone kind so it can be used next time they start their game.

My question is, where do I start. And if any one has any ideas on how bits could be implemented feel free to share it with me. Thanks for any help