Unpack the pieces, and lay them out. Most kits will have a front, back, two side pieces, and 2 roof pieces. Take a look at them, any rough edges can be smoothed out with your fingertips, and check for cracks, those can be fixed with construction royal icing.
Grab a piece of paper and take some notes on the designs you really want, and some things you want to try!
Lay each of the pieces in order that you are ready to decorate. My kit had two roof pieces with a diamond pattern embossed on them, so I piped black royal icing in those groves, then sprinkled yellow and orange jimmies. I did this over a piece of parchment paper to catch the extra sprinkles and reuse them.
Knead the royal icing really well that should come in your kit, it works great at room temperature. Make some more of your own royal icing, which dries really hard for construction purposes. You can also add a color you like.
If you are not going to eat the Halloween gingerbread house, use a glue gun to hold the pieces together.
Once the pieces are decorated and dry, which takes about 8 hours, simply glue them together! That is what I did, and it was SO much easier, but with kids, they might want to eat it.
You can buy some buttercream icing in a tube from the grocery store, but this would be for decorating accents, not to hold or glue the house together.