I'm trying to think of aspects of development and construction that are unique to replica props. On the development side, not a lot; the main one being that unlike almost any other construction project, you will never have complete information. It is always a matter of looking at pictures, making guesses, talking to other people, and in short trying your best to get as close as possible to an unknown goal. Otherwise, it is really like any other project -- often, any other large/complex project. All the development tools from drafting to estimates to milestones are as true for a prop as they are for software development or building a house.
On the construction side, it isn't necessarily so, but prop-making leverages a long history of expedient methods and materials, some of which go all the way back to live theater. They are not unique to props-making, but props are unique in the focus and some of the kinds of application of these methods and materials. Understanding some of them helps in achieving the size and detail of many props whilst staying within limitations of weight and budget and construction time.
Worbla, sintra, foam carving, vacuum forming, hot glue, epoxy clays, bondo skinning, balsa, basswood, acrylic, aluminium, resin casting...and these days add the time savings and surface effects sometimes achievable with laser cutting, 3d printing, pepakura, CNC routing.
You don't need to know all the methods but any one prop is likely to use several methods and materials. And the more you know, the more avenues of approach open up when you are in the planning stage.
If I had to write a guide this morning, on my first cup of coffee, I'd put the essential stages to keep in mind as something like the following:
1) Research. Collect as much as possible. Talk to other prop makers and see what discoveries they may have made. Collect it all in a folder -- don't rely on memory.
2) Drafting. Collate everything you have learned into as close as possible to a single comprehensive design document. This gives you an overview and lets you get a grasp of the basic size and complexity and weight of different elements (allowing you to plan methods of construction), it provides a working reference to keep you within what you researched when you are neck-deep in a sculpt, and it also indicates (far too clearly!) what parts of the prop were obscured by an actor's hand or otherwise unknown and have to be interpolated.
3) Estimates. Before starting to cut, figure out how big it is, how much material it is going to take, how long it is going to take, how much it is going to cost. You don't want to set out only to find out half-way through an essential part won't even fit on your scrollsaw, or the final sword is too heavy for you to lift!
4) Mock-ups. Make mock-ups and massing studies. Any experimental techniques (and even things you know but haven't done recently) you should try out with cardboard, on scraps, etc. Cut out something you can pick up and physically check the scale on. Spray the paint on a scrap and see if it is still good (and is the color you need).
5) Divide and conquer. Split the prop into parts and/or phases. The more you can work on a single part in isolation, the less you will scratch up the finished parts next to it, the less you will mix incompatible materials (like getting aluminium shards in your sculpting clay -- been there, done that!) And even more, it make a project digestible, and gives you a clearer sense of your progress. Oh and yeah; splitting up often gives you such things as ability to take apart for transport, for painting, for adding internal electronics, etc.
Yeah, I'm deep in planning stages for an ambitious prop right now; why do you ask?