Begin with a drawing of whatever you want to create. This will help you create what is called an armature (framework for whatever you’re building).
Imagine seeing through the prop you want to design, what does the skeleton look like basically? What parts go where and how is it all supported? Draw the base structure out clearly so you know what each individual piece will look like and how they fit together in relation. Plan how you will attach your pieces together and how it will all ultimately take the shape of your desired prop.
Choosing your measurements comes next. It can be helpful to do some research and get the actual measurements on the object you are trying to re-create. This makes the object you’re creating read a lot more clearly to the audience. It would not make sense to have a washing machine that is two feet wide, 1 foot deep, and 6 feet tall because you never see them in those dimensions. Choose your measurements, make your cuts, and then assemble your design.
Finally comes detailing, right now you have a very primitive form of your final product. It may vaguely resemble what its supposed to or not at all but the important part is that you now have your foundation. A good detailing job is what turns a wooden box with a hole in it into a washing machine. Using assorted pieces of metal, nobs, and tubing will really make the transformation possible.