This is probably the most tricky part because you have to find an excuse to "scan" her without letting her think that you are up to something exciting (like creating her own personnal piece of lingery).
Just say that you wan't to print a miniature statue of her for your desk, or something that doesn't sound silly. Miniatures are also a cool gift, but it's starting to lack originality. Nowadays, anyone can get a hand on a 3D scanner for less than 300 dollars. Here is the one I used : Asus Xtion Pro Live
, with Skanect
as a capture software and Autodesk Meshmixer
to clean the file.
If you want, you can download the handle I designed to fit my scanner on a Manfrotto Tripod here : http://www.thingiverse.com/thing:212185
ry to get a good resolution from her neck to her hips. This is what you'll need. If she can wear light and skin-tight clothes, it will make your job easier. You can also go for the "Venus" type of statue...
Once you got the scan, do what you told her and print the miniature.
Now that she has no suspicion about your project, the real work begins.
Import the .stl or .obj file into meshmixer and keep only what you need. Try not to deform the model since it has to stay close to reality for your corset to fit perfectly. Smooth the file if it seems a little bit rough and export it again as .stl
Most 3D modelling software will allow you to import a mesh file. You just won't be able to edit it. One thing you have to check carefully is the scale. Sometimes, the STL file will import with the wrong unit, so you have to make sure that 1mm = 1mm and nothing else.
VERY IMPORTANT, or you might have a bad surprise at the end of the project.