To start with, here are the first few steps:
1) Research - Study as many photos as you can on your subject. The more views the better; inside, outside, top, bottom, front, back, sides.
2) Research - Study as many drawings as you can find. Again, top, front, side, and detail views. Print out drawings scaled to the size of your actual model to use as templates.
3) Research even more now... Study other models of your subject to see how others have solved the problems of building a scale model of your subject.
4) By now you will have an idea how to approach building your model. Start with whatever part you feel the most comfortable with. This will build confidence and set the tone for your build.
I hope this helps get you started.

Cheers,
George