I started by scouring YouTube for instructional videos from people who had already built their own. Rifraf's design was my starting point. I loved the way he did the walls, but the top was a bit bulkier and the base less symetric than I preferred. So I melded it with a roof design from another video (which I can't find anymore) and tweaked it so that it would work with a solid core, to keep the wall pieces more stable and well aligned.
Of course, ordering parts from BrickLink can be expensive for small logs (mainly due to international shipping from multiple suppliers), so there are significant economies to scale from building more than one at a time. So I built a whole fleet of TARDISs, giving some away as gifts and making the rest available as kits for those who want to build their own, without having to go through the parts-sourcing hassle.
Build Your Own Lego TARDIS
If you'd like to build your own Lego TARDIS (or assemble one you got from me), I have put the Lego TARDIS instructions on Flickr. If you don't want to scour BrickLink for all the parts, you can buy the kit on eBay.