As Bianca from Closet Historian suggested, the Elizabethan Costume website has a stay pattern generator and tutorial that's great for adapting to 18th Century wear. Today I made up the pattern from the customize-able measurements (exciting!), cut the fabric, did some basic basting stitches, and bought the supplies I'll need next!
I'm not making an Elizabethan costume just yet, as the Georgian era clothing has captured my heart, and I'm not adapting it to another style as that would be to hard for me without a guide, so it may seem not historically accurate... But if it does the job of giving me a flatter, conical-shaped torso, then it's done its job.
Making up the pattern was a pretty fun and easy experience, as I had that big pattern paper that you can get from craft shops. The instructions are very clear and concise from this website, so I'll recommend it.
For my fabric I used whatever I had available, as I was at a loss for something to do today and the sewing shop over the road is closed on Sundays. So I found some light grey velvet-like material (left over from my bonnet) which is quite textured, and some darker grey light suede, which I'll use for the lining, or vice versa. Typically, linen would be used for stays, but I have none available for the time being. Maybe next time.
As for the steel spiral boning and busk, I got them from the Sew Curvy online store, which has a lovely catalog of other stays, some from the Victorian and Edwardian eras! If this corset turns out at least 50% okay I think I'll have a go at an Edwardian corset.
For now we'll see how it goes, as I'm also making a slouchy knit cardigan for my Mum and learning how to knit - so it may take a while!