This is how I do it. I will use a recent example for a mitt used by bowhunters so they can at any moment take their hands out of the mitt and have warm hands on their bow. So a great guy who used to have a successful bowhunting catalog, and invented aspects of the modern tree stand, Paul Brunner, sent me this photo to post online for him:
In this case, Paul sent me this picture, but I could have simply clicked on any picture online and saved it, or I could have made a photo of some object I own in order to undertake a digital record for some subsequent purpose.
You can then import this into some kind of software, maybe there is measurement software, and there certainly are drawing and CAD softwares that you can import photos into. Two free ones you can download somehow, are Inkscape or Draftsight. By import I mean, you either drag and drop the file, or image onto the workspace of the program; or you can simply open the file into the program. I use an old version of Turbocad and it only opens bitmap files (not the usual JPG photos), so to get those I open the picture in Paint, and save it as a bitmap.
Once the file is in the software, a process identical to opening a document in Word, you can proceed to scale it. I usually find it fastest in my software to just drag something, usually a line, square or circle from any two points I want to measure. But the software has measurement tools that are useful for things like irregular areas. These measurements will not be to scale as the picture is not constructed with extractable measurements, but I can get ratios and multiply them out.
Usually though, I will make a cad copy of the picture, which is not complex, but beyond this post. With that, I can scale any part of the drawing to a known measurement, or if I am fitting a chair to my seat, I can adjust the drawing parametrically. So for instance I found a decent plan of a Nakashima chair, that seemed to have adult part weights, which isn't correct, but anyway. I could scale it to a human twice the intended size by just entering 2 in the X and y axis boxes. I can stretch the design any way I want. So in the case of the pattern for the pattern above, I wanted to print one out. So I drew the two straight lines, with four clicks, and the perimeter in the curves function by just clicking around the outline of the shape. Interestingly, I found the shape was not as the numbers implied, which shows the superiority of going from the profile, rather than trying to fit curves to the measurements given. The pattern I ended up with looks like this:
Of course, once you have a CAD drawing, you can use it to print crisp plans; to run CNC machinery; and manipulate it without having to recreate it, or enter parts into your own library of things like knobs that you may re-use. You could for instance copy an interesting case, then adjust it to the golden ratio, for yucks.
With casework, you can easily recreate square shapes with one to two clicks, to any degree of accuracy, the size of the universe or the size of the smallest drawer. Many elements are simply further squares, and most of the ornaments have geometric underpinnings that are easy to model.
Unfortunaely, I can't show you some furniture examples as normally I host those at flcik, which is currently down. I think the bowhunting site would object to my hosting pictures there of furniture.