One of the first things I learned at Le Cordon Bleu was how to make a tart. I used to be so fascinated by perfectly straight and even tart shells, and now I know that one of the important steps is to bling bake the empty tart shell. In order to prevent the bottom from puffing up, you must create ventilation by picking the bottom of the tart with your fork, then cover the bottom with oven-safe cling film or baking paper. On top of that place some pastry weights or rice and beans - their weight will prevent the shells from puffing up, keeping them nice and flat.