To create your own lingerie, you could either buy a commercial pattern, make your own based on an existing garment or custom-make a pattern based on your measurements. The latter is called drafting and requires meticulous calculations. In the interest of convenience, why not work with what you may already have: a piece of lingerie that fits you well. A favourite slip, camisole, bra or bikini can serve as the starting point and pattern for any number of lingerie creations.
Measure your body. In the interest of having a set of reference points when you proceed to make lingerie of all types, take your body measurements. You will not need all these measurements for every project, yet it is still useful to refer to for the particular pieces you are working with. Important dimensions include: high bust (directly under arms), bust (fullest level), under bust (ribcage), waist, high hip, fullest hip (and distance from waist), shoulder to bust point, shoulder to waist, shoulder width, sleeve length and back waist length. Stand in front of a mirror and make sure the tape is level as you measure.
Measure crotch depth. To make custom panties, you will need an additional measurement: crotch depth. Obtain this by sitting on a tabletop or counter and measuring from the side of your body the distance from the bottom part of the waistband over your hip straight down to the flat surface.
Adjust patterns for stretch. Stretchability will determine the eventual size of your pattern. Stretchy fabrics like Lycra-blends will allow you to cut patterns that are smaller than your size, depending on the fit you want. Commercial patterns should recommend which fabrics to use but if you are working with an unfamiliar stretchy fabric you may need several tries before you get the "just-right" fit.
Use something of your own. You need not go through extensive self-measurement if you already have a garment you know fits you well and which you would like to replicate using different materials. Just be ready to part with it, as you will be cutting it to make pattern pieces. Also visualise how you would improve on the garment in the new rendition and make adjustments accordingly.
Make a copy. To make a copy of a favourite panty, cut the seam between the front and back of the undergarment. Cut the front portion first to obtain the crotch portion for the lining. Fold each piece in half at the centre. Place the cut panty pieces on the paper and trace and cut your pattern pieces (three pieces in all: front, back and lining). Lay the pieces flat on the stiff paper. Trace around the panty segments leaving a 1/4-inch seam allowance. Add a 5/8-inch seam allowance at the hips. Now cut and iron your intended fabric into workable pieces and overlay the pattern pieces onto them. Decide which fabrics should go where, if you have a motley collection. Pin the fabric pieces to the pattern and cut and iron the resulting fabric pieces.
Prepare to sew the garment; pin or baste slippery or stretchy fabrics before stitching.