These are just a few basic guidelines to making a corset toile.  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 is ideal.
A marker pen
A seam ripper
Sharp Scissors
Corset Bones
Tailors 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. 
       
Mark up the balance points (notches etc.,) and most importantly the waist line.  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, just sew the two centre front pieces together using the prescribed seam allowance.
       
Press the centre front seems out and sew down the outside edge of each seam in order to form a channel.

	On each side, press the centre back edge in by the seam allowance and stitch 10mm from the edge creating a channel. Sew a re-inforcing line of stitches next to this. Mark eyelet holes at regular intervals 5mm away from the second edge seam on both sides - make sure they match horizontally on each side. Make holes where you have marked with a tailors awl
	Press out the side seam allowances and sew down the raw edges on each side to form bone channels.  Alternatively, you can sew the seam allowance together forming a pocket as shown.  Sew as many of these bone channels as you feel necessary.  Usually one each at the side seams, 2 at the front and one either side at the back is enough for the toile. Insert bones into all the channels and your toile is now ready to try on - You will need somebody to help lace you in.
	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.

     
Fitting

The gap at the back should be 2 inches wide all the way down.  ie: the back edges should be straight.  If they are curved inwards or outwards the corset is either tied incorrectly or it does not fit.

You can adjust minor fitting problems by letting out the seams or taking them in as appropriate.

If you want to reduce the waist, determine how much by - ie: 2 inches then divide this measurement by the number of seams in the corset - excluding the centre front and centre back edges.  Take each seam in by the divided amount at the waist and taper out towards the bust and hip. 

The same principle applies to bust and hip measurements, though these can sometimes be adjusted by side seams alone.

If you make the waist very small for tight lacing, you may need to insert hip gores.  Rip up the side seam to the top of the hip, and mark the width between seams at the bottom edge.  Make a triangular gore for each side which gives 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.