Needless to say both require very different methods. Presuming that you want to make a physical model, there are a number of ways but a good starting point would be to buy a kit; a number of options of which are available on the web at www.stormthecastle.com/paper-castle/make-a-paper-neuschwanstein-castle.htm.
Even if you wanted to make your own from scratch, this would both instruct you on how to do it and provide you with a template to work from. If you wished to make a paper, card, or cardboard model, many of the kits come with shapes on paper, card and cardboard which you then count out and stick together as instructed by the set.
By having the set you could trace these shapes (using transfer paper, available at most crafts stores, if you are working with card or cardboard) and then use the set’s instructions to assemble your 3D castle having added your own design. If you wish to use a computer to put in your own design, simply trace the shape using a pencil and then copy over the line using a thick pen.
Scan the shapes using a computer scanner and then simply add your design. It’s worth using the original stencils in the set as a guide otherwise you may not know what part of the castle you are working on or what you should be designing; after all you do not want to end up with a draw bridge on the roof!