Before I begin, two things I've learned from heartbreaking experience:

1. Don't rush drying, wait at least 24 hours - more if humid - and if you're at all unsure then wait a bit longer.
2. Make spares. You will drop a piece, or stain one, or get impatient and try to pick it up before it's dry - see (1)

Royal Icing construction is excellent fun - if you're the kind of person who appreciates matchstick models or airfix kits anyhow - it allows you to make some fanstastic looking creations which you can make well ahead of time as long as you keep it dry.

First, you're going to need a surface - silicone mat, plastic, or at a pinch greaseproof. Baking parchment will not work - it absorbs water, shrivels and sticks to the pieces. Glass isn't really ideal as it's easiest to get the pieces off the backing by bending the backing.

Next. Icing consistency. For piping outlines and details you need it stiff enough that it holds its shape, but not so stiff that it hurts your hand and bursts the bag as you pipe. For flooding it needs to be runny enough to flow with a little encouragement, but not so runny that it takes forever to dry and isn't strong when it does.

Now colours. Make sure that you make and colour enough icing to do make everything you need - because there's no way you'll manage to match the colours if you find you need to make a second batch. For some details/small areas where you don't mind a bit of colour variation you might want to consider icing the pieces in white and painting after. Here's the same design done in coloured icing, and painted later

Planning your project out using card and sticky tape is invaluable - it lets you fine-tune the shapes of the pieces, and plan out how to build - I usuall number my pieces so I know roughly which order to put things together in. Taking some pictures is handy too. Then I cut along all the sticky tape and either draw around the pieces or lay them directly under my greaseproof so I can trace them.

Remember that the icing is likely thicker than the cardboard you used, so make sure that if any of your items slot together, that you make the slots wide enough for the thickness of icing.

A common hiccup is to make the pieces slightly bigger than the cardboard by icing over the lines.  It's a lot easier to add a little icing if something's too small than remove some if something's too big so remember to ice just inside the lines/shapes as you're piping the outlines.

You can find the recipe here - why not whip up a batch and have a play.