These are just a few basic guidelines to making a corset mock up.  Some of these points are also extremely relevant for making up the real thing.

You will need:

• Plain 'scrap' fabric - a medium to heavy weight calico or plain cotton coutil is ideal.
• A marker pen
• A seam ripper
• Sharp Scissors
• Corset Boning
• Tapered Awl 

Cut your fabric pattern* pieces according to the instructions given in the pattern.  Look at  hints and tips on how to do this HERE

After cutting and before taking the pattern paper off the fabric, number each side of each piece, 1,2,3,4 etc., for one side, and for the other side 1a, 2a, 3a, 4a etc.  This way you wont get confused as to which side is which.  Keep the piles separate and work on one side at a time.  Organisation is KEY to corset making success.  Trust me on this.
        
Mark up the balance points (notches etc.,) if your pattern has them and most importantly mark the waist line on all peices.  NOTE:  The waist is not always marked on the paper pattern - if this is the case, sew as per the instructions and mark the waist in later (see 'fitting' below).
        
It is a good idea to keep the marks on the outside of the toile so that when you eventually try it on, you can see what should be where and what isn't!

Sew the pieces of each side together according to the instructions aligning the waist markings - the finished article should show a nice straight line at the waist.
        
When both sides are sewn together, attach them together at the front.  

There is no need to insert a busk if you are in a hurry, just sew the two centre front pieces together using the prescribed seam allowance, however, you will get a better result if you do include the busk.  All components can be taken out of the toile and used for the proper corset.
        
Press the centre front seems out and sew down the outside edge of each seam in order to form a channel.

The waist line should have a crease running right through it.  If this is the case then the corset is in the correct position.  If the crease appears elsewhere on the corset, either pull it up or down until it feels comfortable - the marked waistline should have the crease running through it, if not, check that it was marked properly.  If the waistline was not marked on the pattern/toile it will be where the crease is.  Mark the toile accordingly.

Note:  On Sew Curvy patterns, the waist is always marked and this is the matching point for all seams.  I always advise that a waist tape is used in the toile in order to avoid stretching if using calico.

You can adjust minor fitting problems by letting out the seams or taking them in as appropriate and where there is strain or space in the corset .. either pin and tuck or slash and spread.

If you want to reduce the waist, do so at the sides and/or back.  If you want to adjust the bust or hip, then these alterations should be done at those points.  In other words, do not do what some patterns advise which is to take the amount you want to reduce/increase by, divide it by the number of seams and then add incremental amounts per seam.  If you do that you will end up with a very very unsatisfactory shape.

If your corset has a very small waist, and large hip/bust spring, then inserting gussets may help with shaping.  Cut a line where you wish the gore to be, insert a peice of calico behind the cut, and pin out a gusset shape in situ to make the extra space required.

Mark all adjustments on the toile and either transfer these to your paper pattern or unpick the toile and use that as your pattern.

it is good practice if using a multi-size pattern, to trace the pattern onto tracing paper thereby preserving a master copy for future use and reference.

Further notes:

• A toile made from plain herringbone coutil will give the best result
• Use a waist tape for accuracy - you can see the waist line, it will feel more comfortable, and stretching while fitting will be minimised
• Use a busk for increased accuracy - or two flat steel bones at the front, depending upon the final effect of the finsihed corset
• Write notes on the toile and take lots of pictures.  It's surprising how much you can see from a picture
• Bone your toile with steel bones for the best results.  These can be recycled in your finished corset.
• No need to use metal eyelets in your toile
• Lacing panels can be recycled for other toiles
• The starting point for sizing your toile is the waist.  Get the waist size on the pattern right and alter the bust/hips around the waist.  it's much easier than trying to make the waist bigger or smaller
• No two bodies are the same, only experience will allow you to eventually predict what might happen when you corset.

On patterns:

• Patterns from commercial companies do not work. This is because they are submitted through a process which adds ease to everything regardless of purpose.  The commercial companies include Vogue, Simplicity, Burda, McCalls.  If you want a proper fitting corset, do not start there.
• DO source patterns from independant companies who specialise in historical costume and corsetry.  There are plenty  of those companies around now and they create proper corset patterns which are designed to modify the body.  These companies include Laughing Moon, Truly Victorian, Sew Curvy and a few others.  At Sew Curvy we do not sell patterns that will make you cry.
• Do not attempt to alter the pattern before making an initial toile.  If you try to do that you will get hoplessly lost.  This is because all pattern makers have a different system.  Measurements will not tellyou the whole story, nor will they tell you how each body will compress in that pattern.  Always make an intial toile according to the pattern instructions, and make alterations afterwards.