The Trials of Dress Fitting
Adjust your dressform to your size. If you, like most women, aren't shaped quite like a standard dressform, you can add bits of padding to map out your fabulous curves. You can also make a perfect copy of your body by creating a custom dressform, using the link in the Resources section.
Find a plain cotton shirt that has as little stretch as possible, unless you plan on making your formal gown out of stretchy fabric. Draw a big line down the middle of the shirt front and shirt back, dividing it equally in half.
Cut the shirt in half along the lines you made. Keep a side, and toss the other. You only need to design on half of the shirt, unless you plan on making an asymmetrical bodice. In that case, don't cut the shirt just yet. You can also cut the sleeves off at this point, regardless of the type of bodice you are making.
Pin the shirt half to the dressform, matching the cut shirt edges with the center front and center back of the dressform. Also make sure that the shoulder seam matches up with the shoulder of the dressform.
Eliminate the bagginess of the oversized shirt so that you have the kind of bodice fit you want. Take in the side seam by pinching the fabric together and pinning it at the side of the dressform. You can also leave the bagginess if you want a loose fit. If you need to, also take in the shoulder seam. Draw the new shoulder and side seams onto the shirt.
Rummage through your closet for some inspiration, even if you already have a design in mind. Look for garments that have elements of your ideas in them already, such as necklines, sleeves, skirt styles and waistline heights. These will give you a preview of what styles already look good together, as well as show the levels of construction difficulty.
Draw the waistline you want directly onto the shirt. Most formal gowns will have the waist at the natural waist, hip or empire level. Keep in mind, though, that you are not a dressform and that your body will fill out the bodice more. This is where your other clothes can come in handy. Compare the waist level with that of a comparable garment so that you know you have the waist at the proper height for you. Cut the shirt along this line.
Draw the neckline you want directly onto the shirt. Keep the advice in Step 7 in mind, as a too-low neckline can be a big "uh-oh" later on. Cut the shirt along the neckline.
Sassy Sleeve Design
Dig those discarded sleeves out of the scrap pile, and find the match for the side of the shirt you are working on. Pin the armhole edge to the shoulder of your dressform, matching the shoulder and side seams. If you don't want sleeves on your formal gown, ignore this section entirely.
Take in the side seam of the sleeves to match the bodice armhole, which will have changed if you adjusted the side seam of the bodice. Make sure that the width of the sleeve still fits your arm.
Cut the sleeve short to get the length you want. Generally, formal gowns today have short sleeves, either just capping the shoulder or ending at the elbow. You can also keep the sleeve long.
Adjust the fit of the sleeve on your own arm. You may need the aid of a friend for this one or at least another garment with which to compare the sleeve. Make sure that you can move your arm freely. If you need to add width, for a flowy or gathered sleeve, cut the side seam apart and pin some extra fabric there. Draw in the new side seam.
Take some notes on the sleeve and bodice construction so that you remember what you planned. If you will need to gather the sleeve hem, waistline or neckline, make a note of it.
The Bottom Half
Search the closet again for an appropriate skirt style. Formal gowns usually have long skirts, knee to ankle length; although, obviously, you can make your dress as short as you want. Select a dress or skirt in the style you need.
Trace the skirt panels onto a big sheet of muslin fabric. At this point, if there are any style changes you would like to make, such as length or width, make them. Cut out the panels, and then pin the side seams together. Also, draw lines at the center front and center back of the skirt.
Pin the waistline of the muslin skirt to the waistline of your dressform, matching the center front and center back to the dressform. Make sure that the waistline fits the bodice and that there are no sizing problems. This is your last chance to make any changes to the skirt design.
Make sure that all seamlines are clearly marked and cut. Also mark each dress piece like a pattern piece so that you don't mix them all up later. At each seamline and hem, write the amount of seam allowance you will need. Add these seam allowances when you cut the final dress pieces from your dress fabric.
Take the dress apart. Use the dress pieces like you would pattern pieces for any dress, and sew the dress together.