The Awesome Platinum Simmers have created a super awesome guide to the new Blueprint Mode in The Sims 3! Check it out below!

Platinum Simmers and the New Blueprint Mode!

Can you use Blueprint mode to build an entire house?
Yes, although it’s probably not as easy as you’d think. Think of a jigsaw puzzle, without an actual solution! You can place a selection of blueprints down and move them about until you find something you think works. Preview, check, and if you don’t like your results, cancel and start again.
Even once you’ve got a layout that works, you’ll still need to tweak it. Blueprint mode doesn’t snap doors and windows together; it’s up to you to make sure that you can get into all the rooms and that doors and windows match and flow. However, the dropper tool makes this a doddle.