Organization is the first step to good costume construction.
Before any actual seaming and finishing can begin several steps
in fabric, pattern, and notion preparation must be done:
PREPARING THE FABRIC
Preshrink all fabric and notions (with the exception of thread
and closures) before cutting. Washable fabrics should be washed
as indicated on the manufacturer's instructions. If a fabric can
be hand washed or dry cleaned only, then this process should be
completed prior to cutting. Notions such as trim, zippers, and
some interfacings should also be preshrunk.
Straighten fabric ends to coincide with the crosswise threads by
snipping close to the selvage and pulling a crosswise thread
until it puckers across the width of the fabric. Cut along the
puckered line, this gives you a straight grain to begin working
from. Some sturdy fabrics may be ripped across the width to find
the crosswise grain. This ripping process must be a quick, smooth
motion, or the fabric will rip down the lengthwise grain as well.
Not all fabrics will tear, so the most accurate, albeit the most
time consuming method, is to pull a thread.
Marking ends of
fabric
Fabric is on grain when the crosswise and lengthwise threads
are at a perfect right (90 degree) angle to each other. To
determine if your fabric is on grain lay it out on a cutting
table aligning a large corner of the fabric with the corner of
the table. If the corners do not match, the fabric must be
realigned. If the fabric is only slightly off grain fold the
fabric in half, pinning the selvages together every two or three
inches and steam press threads into proper alignment.
Checking the
squareness of the fabric 
Straighten fabric by pressing 
If your fabric is very off grain, it can be straightened by
stretching and pulling the fabric in the opposite direction from
the way the ends slant until a perfect right angle corner is
formed.
Straighten Fabric
by pulling 
Lightly press the fabric to remove the bolt fold line and any
wrinkles present.
PREPARING THE PATTERN
Sort all pattern pieces necessary to construct the chosen
garment. If using a commercial pattern, cut the individual
pattern pieces out along the cutting line.
Group all pattern pieces for lining, interfacing, etc. together.
Make any and all alterations to the pattern necessary for the
proper fit. This process will usually have been done for you when
using original costume patterns.
Press all pattern pieces flat with a dry iron.
After all of the above steps are completed, you are ready to
begin laying out the pattern pieces in preparation to cutting the
garment. Keep the guidelines below in mind as you proceed:
Lay out all pattern pieces before cutting. Commercial patterns
have a lay out guide for each view and size. Consult this before
beginning. Cutting before all pieces are correctly laid out may
result in a shortage of fabric. It is far better to have this
information before any cutting begins, as it can usually be
remedied by using alternate layouts.
Place pattern pieces printed side UP unless otherwise indicated.
Pin first the lengthwise and crosswise grainlines and foldline.
Each grainline must be checked by measuring from the grainline to
the selvage edge of the fabric at top and bottom of the
grainline.
Place the pins perpendicular to and about 1/4" inside the
cutting line. At corners, place the pins diagonally. Space the
pins two to three inches apart, closer for very slippery or sheer
fabrics.
As often as possible, cut directionally with the grain.
Never lift the fabric from the table. Keep one hand flat on the
pattern piece while cutting.
Cut the garments out with long, steady strokes.
Cut each pattern piece the correct number of times. Pockets,
facings, cuffs, welts, etc. may need to be cut multiple times.
Save fabric scraps from cutting, these pieces may be necessary
for plackets, buttonholes, etc. or can be used to test tension,
press-ability, etc.
Fold the cut pieces softly and lay on a flat surface. Do not
bunch the pattern and fabric pieces up, as the pattern may come
off the pattern before all markings have been transferred.