The above answer is a little short and basic. Basically, you get a drawing or picture of whatever you want to model. The more drawings/pictures the better. Then try to break the part down to sub assemblies. When you build it don't be afraid to mix plastic, metal or any other product that will work for what you're trying to model. Make the part to be modeled as detailed as you wish or have the ability to do. You'll find that once it's painted all those little pieces will look like they are one complete part. Hope this helps.
Any ship larger than a Destroyer is a waste of metal.