Put on t-shirt or whatever you are going to wear. If you are doing a full over-the-breast corset, wear a bra - a few sizes too small even - the closer it fits you to the end result you want in the corset the better. If you know there are any body parts that aren't going to be covered by what you're wearing (hips can be a problem, and tape adhesive can be hard to get off clothing), put on lots of baby powder. Wrap a strip of duct tape over your shoulder, down through your legs, and back up and over your shoulder again. Make it fairly tight. Do it on both sides. The point of this is that there will be a lot of tension at the waistline and everything will have a tendency to collapse and bunch in. This creates vertical tension which minimizes the bunching, and makes it easier to fit a better pattern. With the tape still on the roll, hold the very end of the tape, tighten your stomach in, and wrap the tape around you once as tightly as you can...tighter than that if you want a dramatic waist. Slightly overlap the ends of the tape and cut. Try to keep the tape as smooth as you can. Overlapping first strip half an inch or so, repeat , working up body. KEEP SUCKING IN! if you want a dramatic waist but room to breathe, expand your rib cage slightly as you work up. Keep in mind this will essentially be the deepest breath you will be able to take when wearing the corset. If you want a shorter style, wrap the tape flush against your body. If you're making a full corset you can either take the curve of the waist up to the widest part of breast in a rather smooth line, or you can contour the shape more to your body.
Bring the tape an inch or so beyond where you think you'll want it the edges of corset to be. With exception of the overlapping between the strips there should only be one or two layers of tape. There will probably be a bit of bulging between the overlapped areas at this point.
Now begin working from waist down. You probably want to keep it fairly loose over the sides and back, and a bit tighter over stomach. Keep it loose at the area you want the bottom edge to be, especially over the rear, or it may cause you to bulge unpleasantly around the edges when you're laced in. Again, extend about an inch beyond where you want the edge.
If you are making a corset with a very small waist you may want to tape strips of boning on either side to support and keep a smooth line. Don't plan on being able to remove the boning from the pattern later!
After you have a base layer of tape, start shaping. The closer the tape molds you to how you want the corset to fit, the better. Smooth the bulges by pulling tape tightly over these areas, and tighten areas which are too loose. Try to be sparing with the amount of tape you're using.When you're done you want it to be at least three or four layers thick, but avoid getting it more than seven or eight if possible. Wrap the tape so that it follows the contours of the body without much wrinkling or folding. If you are doing an over-bust corset and you want a pushed-up effect, make some adjustments to be sure you are pushed up! Also try to keep both sides fairly symmetrical. They probably won't be exactly the same, but the closer the better if you want to be certain about how the finished corset will fit. You will only be using one side to make this pattern, so don't worry too much about perfecting both sides unless you are going use the other side to make another pattern at the same time. You may want to do some vertical strips to finish smoothing it out. If a part has been wrapped too tightly (particularly at rear, hips or under arms), and you want it looser, carefully cut a slit in the tape, attach new piece of tape to one side, pull in to desired fit, and attach end of tape on other side.
The seam placement, center front, center back markings, and matching lines are done most easily by marking the edge of the narrow masking or scotch tape with a china marker, and laying this down where you want it. Lay strip of tape lengthwise down edge of table or door and run marker down it for a sharp, straight line.
First mark center front and back. Be very certain it's perfectly centered and vertical all the way down. This is a perfect opportunity to screw up the pattern if you're not careful. The friend comes in quite handy for doing the back. If you don't like where you put a line, just peel it off and try again.
If you want to make the waist of the corset smaller than you can actually wrap yourself you will need to mark the smallest part of the waist so you will know where to make adjustments later. Wrap a tape-line around smallest part of waist from front to back. The front will be lower than back. Be sure it's a fairly smooth line, although the sides will probably have a greater rise than the front or back.
Next lay the seam lines. The space you have between each seam should be about the same all the way around waist line, although the back panel which will have the lacing panel may be wider. Especially if doing a fan or four panel style the tops and bottoms will be much different in width, but the width of panels at waist is about the same. The diagonally angled panels on these may cause solid boning to twist if they curve to the side too much. If you already have flat boning, lay it over line and press it down over the whole length. If it seems to lie flat and stay over the line it's probably ok.
Now each panel must be marked with a grain line. This will be used to orient the pattern piece on the fabric when you transfer the pattern. Most patterns use a vertical grain line, but for a corset the horizontal grain is what is crucial. If you already marked the waistline do it just above this. If you didn't mark the waistline, begin by placing the tape at the smallest part of the waist on the side. As this is where the greatest tension will be, this area needs to be perfectly on the grain (the fabric will stretch on the bias if the pieces aren't cut properly with the grain of the fabric, and if it is even a little off it can cause problems which may not be evident immediately, as the fabric may stretch with wearing). Then bring the tape to the front, and then around the back, keeping line perfectly horizontal. At the front the line may be above the smallest part of the waist, and it may be below the smallest part of the waist in back.
Finally, mark the top and bottom matching lines. I usually space them farther apart in the center front and back and bring them closer in the middle, following curves of body. This makes it more obvious if you accidentally get your panels switched around or upside down when you start sewing. Once you are more experienced you may choose just to use the grain line as the matching line
If you have your busk already you should mark the top and bottom of the busk, and allow for corset to extend about 1/2 inch or more beyond busk ends.
The top and bottom lines are done the same way. It can be a little more difficult getting the tape to curve the way you want here, so you may want to use short sections, or use narrow electrical tape, which curves more easily, here.
Finally, CAREFULLY cut yourself out. Bottom to top is easiest for me. Work your hand up underneath the tape to lift it off of your flesh and cut line as accurately as you can . When you've peeled it off, immediately put powder on any parts where adhesive may be exposed to keep it from sticking to itself. Then cut up the center back line.
Number the pieces from front to back, and draw directional arrows on waist line to show which way they will be stitched together. The more panels you use the harder it is to tell after they've been cut. ..
You will often find as you fit the pattern that the stomach was a little loose and the rear and hips a little tight.
To remedy the stomach you may want to remove a long narrow triange a quarter inch wide or more at the bottom, and tapering to waist in the front. To give a little room over the hips and rear, add 1/8" to 1/4" to the bottom width of the back-side piece(s), and taper in to the waist.
As you gain more experience with pattern-making you will be able to devise other ways to adjust the fit.