To glue pieces, either building itself or decoration, you will need support. Use glasses, vases, boxes, etc.
Gingerbread pieces/cookies for gingerbread house requires a week at least to firm up after baked, dried up after being glued. This will resulted in strong standing building. Some people make their gingerbread house a month or two in advance even.
For window, I glued gelatin sheet, so they are see through, instead melting candies.
On the inside window pane, the top parts, I sticky-taped a battery operated light on each. And on these windows, I showcased tiered cakes and pastries. The lights added extra touch for sure IMO!
And because this is a dollhouse type gingerbread house, the wall on the back is open, thus I glued all decorations before adding roof.
Now for the roof. They are using wafer sheets instead gingerbread cookies. You can use gingerbread cookies obv. so here is the NOTE: double the recipe as the one I typed is only enough for front wall and sides, that is why the floor of the house is made of gingerbread clay instead cookie :) But of course, this is depending the size of your gingerbread building
Glue roof to structures with royal icing. Spread royal icing on board and cover with coconut. Glue outside decorations same as inside, with royal icing that is. Make royal icing drizzled here and there as snow. Glue garland on roof also with royal icing. I also added a little cotton on the outside fire place (chimney) to create a "smoke".
Basically decorate as you wish and have fun :)