The term "corset" didn't really come into use until the 19th century. I'm still using it because the contemporary terms "(pair of) stays" or "pair of bodies" are not well known in our day. All stays from the 17th until the late 18th century have basically the same shape and follow the same basic construction method. The differences are in the details. So before you begin, you have to make a few decisions as to what kind of stays you want. This will decide what fabric and boning you need and how much of it.
N.B.: Please read all chapters of the following instructions before you begin. Better yet, before you buy the fabric. You may not understand everything upon the first reading, but that is normal: In my experience, you usually don't really understand sewing instructions until you've got the fabric in your hands and have arrived at the stage described. That's normal. However, if there's something you still don't understand while actually working on your stays, please let me know. I'm trying to make everything as clear as possible, but I need feedback in order to do so.
One of the main difficulties when making stays is the correct fit: The more parts the pattern consists of, the more difficult it is to adjust it. For a proper fitting, you first have to put all the boning in and make the lacing holes, i.e. you have to almost finish the stays before you know whether stage one (i.e. the pattern) was correct. Therefore, buying a good commercial pattern may make sense if you can spare the money more easily than you can spare the time for making a mock-up, but I wouldn't recommend relying 100% on even the best commercial pattern.
In any case, it makes sense to make a mock-up following method 2.1 above: Machine-sewn, without top fabric, lining, or neatened edges. Apart from checking the fit, you can find out whether you've put in enough boning, and the right kind of boning, if you wear the mock-up for a few hours. If all is well, you can add top fabric and lining and neaten the edges to make proper stays out of it. If you need any more incentive for investing effort into a mock-up, remember that properly-fitting stays are the basis on which all your further 18th century clothing projects depend. If the stays don't fit properly, the robe won't fit, either. Moreover, ill-fitting stays will sooner or later hurt. Most people who complain about how uncomfortable or even painful stays are (including the dialogue writer for "Pirates of the Carribean", may he burn in hell) simply haven't worn well-fitted dtays yet. A mock-up that has only slightly gone wrong can always serve to give your modern dress dummy a corseted shape, thus facilitating subsequent robe fittings.
The following instructions are based on a 1770s pattern that is well suited for beginners in that it is easy to cut, relatively easy to adjust in size, can quickly be made up with the machine and doesn't need a lot of boning. The pattern is still authentic, though: When you're sure it fits, you can make it up by hand and voilà, authentic stays. If you're only concerned with the shape they give, they can be used from ca. 1690 until 1790.