There are two typical approaches (that I'm aware of, anyway):
The first approach is pretty straight-forward; take some suitable drafting paper, a pen or pencil, and a long ruler.
After doing sufficient research online of one's subject of interest, start out with drawing a large square/rectangle/circle (depending on the general shape of the prop) and then being tracing out a rough shape within that.
I typically go through three to four rough drafts, making revisions to certain areas, details and proportions as I go along. Use whatever reference pics you have to make any necessary changes to the sketch.
The second approach is admittedly something I've never had any experience with. There are various software programs available (ie. AutoCAD, Corel Draw) for digitally drafting blueprints.
Some software is designed to generate digital blueprints in 2D only (ie. profile views), whereas other software allows one to create a 3D blueprint which is useful for CNC-ing items.
Keep in mind such software is never cheap, so compare between the different features to find what you're looking for. Hopefully you'll choose one that's a little less expensive.
That's putting it in laymen's terms, but I hope that explains it somewhat.