Grease a baking sheet, or cover the bottom with non-stick baking paper (baking paper is probably better if you are using sweets in the windows). Roll the dough out so that it is 3-4 mm thick.
When cutting out the shapes it is often best to do it on the baking sheet so that they don't get distorted when you move them (though you can see from the pictures I forget to do this)
I find it best to press a ruler into the dough to get straight edges. I use a protractor to get them at the right angle. I make crenelations by using the edges of cookie cutters.
With the left-over dough you can make gingerbread trees and animals or a fence to place round your building. Last year one of my junior assistants made an army of gingerbread men to defend my castle. I sometimes also make a few spare parts for the castle in case any get broken.